It doesn't matter if you only need to use the thing once - you're using a third-party unofficial (and illegal in some countries) device to get whatever exploit in the first place. As I had previously implied, if you can't get the event on a retail, unmodified game and console, then it's not legitimate. You know, because obtaining something that can't be obtained without hacking is totally legitimate, right? All it takes is one use of third-party unofficial hardware to get non-JP Mew from Emerald, or Hall of Origin Arceus, etc.
And yes, Gateway is a pirate cart. Unlike DS flashcarts, it was sold as a ROM loader from the beginning, offering no native means of booting homebrew, or even ROM dumping support. You can't even fall back onto the excuse of dumping carts you physically own and playing off of the backups because VERY few people had the capability to dump their 3DS games before 2.0. People had to modify their booting program to get anything out of it other than piracy.
Importing retail JP carts of previous gen Pokémon games is completely different, since 1) previous generations didn't track your DS region, 2) it's an actual, official game cart, and 3) an older DS console, which does not have region-locking, can play those games/be used to obtain those events. Or did you defeat the region-locking on your DSi for 5th gen games somehow? In that case, a purist would say they're not legitimate.
Since this event appears to be unable to be obtained on non-JP games without some sort of unofficial modification, it doesn't look good for the upcoming JP Diancie event.