theSLAYER

A rudimentary viewer and editor program for GP1 files.
Supports drag and drop (just drag the GP1 onto the program).

Has a function to store location data, as well as use stored location data.
The bundled location.txt needs to be in the same folder as the program, for it to work.
Also has in-built CP calculator, based on IVs and level.
Note: the CP shown, is based on PGo CP calculation, which is how Go Park works.
Initial encounter, it will show PGo details. Once captured, the CP gets rerolled to Let's Go CP.

If you're rehosting it, please link/source THIS page.
Source Code: https://github.com/PP-theSLAYER/gp1editor
(I gotta warn you, it's horrible code)

Note from 13th December, 2018:
On this day, some of the stats of certain Pokemon has been changed.
If I'm not mistaken, unlike the forced stat change enforced during Let's Go's release, these only kick in if:
(1) you evolved into that Pokemon after the change or
(2) you powered up the particular species after the change or
(3) you freshly captured that species after the change or
(4) you received the Pokemon from trade after the change.
Meaning if you already had the creature before the change, it's CP and stats will remain unchanged on Pokemon Go, until you performed one of the points above.
Do note that these changes could be enforced in the future, even if the actions specified above wasn't performed.
[Confirmed on my end; I transferred 2 Meltan on 13-Dec, one captured before and one captured after the change.
The older one uses old CP calculation, while newer one uses new calculation. See image in screenshots section]

 In any case, these are the Pokemon affected by the change:
 

Distribution Title ポケモンスクラップ2016　マスターボール Wondercard ID 158     Games XYORAS Type Serial Code Distribution Date April 28 - August 31, 2016 Item Master Ball Quantity x1 Receiving Text 0x35 Thank you for receiving this distribution! Please pick up your gift from the delivery girl in any Pokémon Center. Distribution Text ポケモンスクラップを　２０まい
あつめた　ごほうびに
マスターボールを　プレゼント！！

Credits to Kirzi for this hard to obtain Birthday Pokemon.
Distribution Title おたんじょうび　おめでとう！ Wondercard ID 184     PID Cannot Be Shiny Games XYORAS Type Local Wi-Fi Dates Oct 1 -
Dec 31, 2016 Species Eevee Nickname (default) Gender Male Level 10 Nature Random Ability Run Away (1) or Adaptability (2) OT ポケセン♪ TID 10016 Location a Pokémon Center Ribbon Birthday , Ball Cherish Ball Held Item None Moves Celebrate Tail Whip Baby-Doll Eyes Swift Relearn
Moves Celebrate (None) (None) (None) Receiving Text 0x07 Thank you for coming to the Pokémon Center! Please pick up your gift from the delivery girl in the game.  

This Arceus was distributed to HKTW players who attended the Movie Hoopa and the Clash of Ages via Serial code.
Distribution Title アルセウス　プレゼント Wondercard ID 123 PID Shiny Locked PID Games XYORAS Type Serial Code Dates Nov 20 - Dec 17,
2015 Nickname (default) Gender Genderless Level 100 Nature Random Ability Ability 1 OT デセルシティ TID 3075 Location Pokémon Movie 15 Ribbon Wishing , Ball Cherish Ball Held Item Silk Scarf Moves Judgment Blast Burn Hydro Cannon Earth Power Relearn
Moves Blast Burn Hydro Cannon (None) (None) Receiving Text 0x03 Thank you for coming to see the 2015 Pokémon Movie! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title トレッタから　ぬけだした　ソーナンス Wondercard ID 4     PID Cannot Be Shiny     Games XY Type Infrared Dates Dec 15 2013,
Dec 23 2013,
Jan 19 - 26, 2014,
Jan 29 - Feb 16, 2014 Species Wobbuffet Nickname (default) Gender Male Level 10 Nature Random Ability Shadow Tag (1) OT ヌケッタ TID 12063 Location a Pokémon event Ribbon Event Ball Cherish Ball Held Item None Moves Counter (None) (None) (None) Relearn
Moves (None) (None) (None) (None) Receiving Text 0x2B Thank you for playing Pokémon! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title ウチダアツトの　ピカチュウ Wondercard ID 33     PID Cannot Be Shiny     Games XY Type Local Wi-Fi Dates Jun 15 2014 Species Pikachu Nickname (default) Gender Male Level 22 Nature Random Ability Static (1) OT ウチダアツト TID 22 Location an event site Ribbon Classic Ball Cherish Ball Held Item Light Ball Moves Quick Attack Electro Ball Double Team Mega Kick Relearn
Moves Mega Kick (None) (None) (None) Receiving Text 0x50 Thank you for coming to the event! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title トレッタに　もぐりこんだ　ロトム Wondercard ID 45     PID Cannot Be Shiny     Games XY Type Local Wi-Fi Dates Oct 15 - Dec 3, 2014 Species Rotom
(Normal) Nickname (default) Gender Genderless Level 10 Nature Quirky Ability Levitate (1) OT トレッタ TID 9254 Location a lovely place Ribbon Classic Ball Cherish Ball Held Item None Moves Shock Wave Astonish Trick Thunder Wave Relearn
Moves (None) (None) (None) (None) Receiving Text 0x2B Thank you for playing Pokémon! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title メガトウキョーの　リザードン Wondercard ID 139     PID Shiny PID     Games ORAS Type Local Wi-Fi Dates Jan 1 to
31, 2016 Species Charizard Nickname (default) Gender Male/Female Level 36 Nature Random Ability Blaze (1) OT メガトウキョ TID 12124 Location a Pokémon Center Ribbon Premier Ball Cherish Ball Held Item Charizardite Y Moves Overheat Solar Beam Focus Blast Hold Hands Relearn
Moves Hold Hands (None) (None) (None) Receiving Text 0x07 Thank you for coming to the Pokémon Center! Please pick up your gift from the delivery girl in the game.

Distribution Title いろちがいのレックウザ　プレゼント Wondercard ID 140     PID Shiny PID     Games ORAS Type Local Wi-Fi Dates Feb 1 to 29, 2016 Species Rayquaza Nickname (default) Gender Genderless Level 70 Nature Random Ability Air Lock (1) OT コロコロ TID 3145 Location an event site Ribbon Event Ball Cherish Ball Held Item None Moves Dragon Pulse Twister Thunder Extreme Speed Relearn
Moves (None) (None) (None) (None) Receiving Text 0x2B Thank you for playing Pokémon! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title フーパ　プレゼント Wondercard ID 124     PID Cannot Be Shiny     Games XYORAS Type Serial Code Dates November 3 to
30, 2015 Species Hoopa Nickname (default) Gender Genderless Level 50 Nature Random Ability Magician (1) OT Mighty TID 7185 Location Pokémon Movie 15 Ribbon Wishing Ball Cherish Ball Held Item Focus Sash Moves Hyperspace Hole Psychic Astonish Nasty Plot Relearn
Moves (None) (None) (None) (None) Receiving Text 0x03 Thank you for coming to see the 2015 Pokémon Movie! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title ボールのもようの　ビビヨン Wondercard ID 36     PID Cannot Be Shiny     Games XY Type Local Wi-Fi Dates Aug 1 -31 2014 Species Vivillon
(Poke Ball Forme) Nickname (default) Gender Male/Female Level 12 Nature Random Ability Shield Dust (1) or Compound Eyes (2) OT ポケセン TID 8014 Location a Pokémon Center Ribbon Classic Ball Poke Ball Held Item None Moves Stun Spore Gust Light Screen Struggle Bug Relearn
Moves (None) (None) (None) (None) Receiving Text 0x07 Thank you for coming to the Pokémon Center! Please pick up your gift from the delivery girl in the game.

Distribution Title ラティオス　プレゼント Wondercard ID 106     PID Cannot Be Shiny     Games XYORAS Type Local Wifi Dates Aug 31 - Sep 6, 2015 Species Latios Nickname (default) Gender Male Level 50 Nature Modest Ability Levitate (1) OT ポケセン TID 8315 Location a lovely place Ribbon Wishing Ball Cherish Ball Held Item Latiosite Moves Dragon Pulse Luster Purge Psychic Heal Pulse Relearn
Moves Dragon Pulse Luster Purge Psychic Heal Pulse Receiving Text 0x35 Thank you for receiving this distribution! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title ルギア　プレゼント Wondercard ID 105     PID Cannot Be Shiny     Games XYORAS Type Local Wi-Fi Dates Sep 7 - 13, 2015 Species Lugia Nickname (default) Gender Genderless Level 50 Nature Timid Ability Pressure (1) OT ポケセン TID 9075 Location a lovely place Ribbon Wishing Ball Cherish Ball Held Item Leftovers Moves Aeroblast Hydro Pump Dragon Rush Ice Beam Relearn
Moves Aeroblast Hydro Pump Dragon Rush Ice Beam Receiving Text 0x35 Thank you for receiving this distribution! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title ピカチュウ　プレゼント Wondercard ID 103     PID Cannot Be Shiny     Games XYORAS Type Local Wi-Fi Dates Aug 24 - 30, 2015 Species Pikachu Nickname (default) Gender Male Level 50 Nature Naughty Ability Static (1) OT サトシ TID 8245 Location a lovely place Ribbon Wishing Ball Cherish Ball Held Item Light Ball Moves Thunderbolt Quick Attack Iron Tail Electro Ball Relearn
Moves Thunderbolt Quick Attack Iron Tail Electro Ball Receiving Text 0x35 Thank you for receiving this distribution! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title ヤンチャム　プレゼント Wondercard ID 104     PID Cannot Be Shiny     Games XYORAS Type Local Wi-Fi Dates Aug 17 - 23, 2015 Species Pancham Nickname (default) Gender Male Level 30 Nature Adamant Ability Mold Breaker (2) OT セレナ TID 8175 Location a lovely place Ribbon Wishing Ball Cherish Ball Held Item Black Glasses Moves Arm Thrust Stone Edge Dark Pulse (None) Relearn
Moves Arm Thrust Stone Edge Dark Pulse (None) Receiving Text 0x35 Thank you for receiving this distribution! Please pick up your gift from the delivery girl in any Pokémon Center.

Distribution Title ポケモンカフェの　ピカチュウ Wondercard ID 79     PID Cannot Be Shiny     Games ORAS Type Local Wireless Dates Jan 9 - Mar 15, 2015 Species Pikachu Nickname (default) Gender Male / Female Level 10 Nature Random Ability Static (1) OT Ｐ－Caｆｅ TID 1095 Location a lovely place Ribbon Special Ball Cherish Ball Held Item Sweet Heart Moves Play Nice Charm Nuzzle Sweet Kiss Relearn
Moves (None) (None) (None) (None) Receiving Text 0x2C Thank you for coming to the store! Please pick up your gift from the delivery girl in any Pokémon Center.

Players that watched Pokemon Movie M17: Diancie and the Cocoon of Destruction in Japan,
have access to a free downloadable game, and completion of the game will give a serial code to a Master Ball.
 
WC 23: マスターボール　プレゼント Master Ball
        x 1 0x02 Thank you for coming to see the 2014 Pokémon Movie! Please pick up your gift from the delivery girl in any Pokémon Center.                         Serial Code  |  Jul 19 to Oct 15, 2014  |  JP 3DS: XY マスターボール　プレゼント ポケモンえいがを　げきじょうに
みにきてくれて　ありがとう！
やせいの　ポケモンを かならず
つかまえる　ことが できる
マスターボールを　プレゼント！

Players that were present at the Jump Festa 2014 in Japan,
can receive the Weakness Policy event via Local Wireless.
 
WC 6: じゃくてんほけん　プレゼント Weakness Policy
        x 1 0x2B Thank you for playing Pokémon! Please pick up your gift from the delivery girl in any Pokémon Center.                         Local Wireless  |  Dec 21 to 22, 2013  |  All Regions 3DS: XY じゃくてんほけん　プレゼント ジャンプフェスタ２０１４ に きてくれた
きねんに ３れんしょうバトルで
かつやくする ための どうぐ
じゃくてんほけん を プレゼント！

This gift was traded to players during a Christmas Celebration.

Players will select Gengar or Scizor, and the a Haunter or Scyther (w/ Metal Coat) is traded to the players,
and it will evolve upon receiving it.

This page also includes a simulated card, for the Haunter and Scyther.

The Christmas Lucky Egg code can also be received at this event.

It appears this trade event had a season 2 due to popular demand.

12x Quick Balls distributed via Serial code, for purchasing the Digital Copy of Pokemon Sun and Pokemon Moon.
Event was Available to EU, JP and TW (via EU/JP 3DSes).
 
WC 1602: ダウンロードばん　とくてん Quick Ball
        x 12 0x36 Thank you for receiving this distribution! Please pick up your gift from the deliveryman in any Pokémon Center.             Serial Code  |  Nov 18, 2016 to Nov 30, 2017  |  JP: SM ダウンロードばん　とくてん せんとうが　はじまって　すぐだと
ポケモンが　つかまえやすくなる
クイックボールを　１２コ　プレゼント！

100x Poke Balls distributed via Serial code, for purchasing the Dual Pack Retail Copies of Pokemon Sun and Pokemon Moon.
Event was Available to US, KO and JP
WC 1603: ダブルパック　こうにゅうとくてん Poke Ball
        x 100 0x2C Thank you for playing Pokémon! Please pick up your gift from the deliveryman in any Pokémon Center.             Serial Code  |  Nov 18, 2016 to Nov 30, 2017  |  JP: SM ダブルパック　こうにゅうとくてん やせいの　ポケモンに　なげて
つかまえる　ための　モンスターボールを
１００コ　プレゼント！
このボールで　たくさんの　ポケモンを
つかまえよう！

This wonder card is sent from the Demo to your main Pokemon Sun or Pokemon Moon game.
WC 2045: とくべつばん　すいせいのかけら Comet Shard
        x 1 0x5B Thank you for playing the demo version! Please pick up your gift from the deliveryman in any Pokémon Center.             Pokemon Demo  |  Any Date  |  All 3DS: SM

This wonder card is sent from the Demo to your main Pokemon Sun or Pokemon Moon game.
WC 2044: とくべつばん　かおるキノコ Balm Mushroom
        x 1 0x5B Thank you for playing the demo version! Please pick up your gift from the deliveryman in any Pokémon Center.             Pokemon Demo  |  Any Date  |  All 3DS: SM

This wonder card is sent from the Demo to your main Pokemon Sun or Pokemon Moon game.
WC 2043: とくべつばん　きれいなハネ Pretty Wing
        x 10 0x5B Thank you for playing the demo version! Please pick up your gift from the deliveryman in any Pokémon Center.             Pokemon Demo  |  Any Date  |  All 3DS: SM

This wonder card is sent from the Demo to your main Pokemon Sun or Pokemon Moon game.
WC 2042: とくべつばん　きんのたま Nugget
        x 1 0x5B Thank you for playing the demo version! Please pick up your gift from the deliveryman in any Pokémon Center.             Pokemon Demo  |  Any Date  |  All 3DS: SM