Intro
Since there isn't any information posted about how the C-Gear is planted within the save file, I might as well just explain how one would edit it. PokeStock is capable of editing the skin, but it's a pretty weird standalone program -- people may want to have their own program (in english) that can edit it. Plus it's good to know how it works!
Background Information
The C-Gear Skin is stored inside the save file as a .psk file ("poké-skin") after it is downloaded or edited. It's a pretty big portion of the save file, coming in at 9.50 KB of data -- that's around 2% of the entire save file's size. Visualizing it as an actual image is different, the .psk file format is not a .bmp or .jpg, and this is beyond me... PokeStock is able to visualize it and save it as a view-able image, however.
Documentation of Save Structure for C-Gear Skins:
0x52000-0x545FF
Contains the .psk data.
Calculate Checksum of this region, call it checksum(1).
0x54603-0x54604
Contains the checksum of the .psk data.
Overwrite the current value with checksum(1).
0x1C024-0x1C025
This is a Checksum Reference to the .psk checksum.
Overwrite the current value with checksum(1).
0x1C02E-0x1C02F
This is a Region Checksum of the reference checksums, which must be recalculated.
Calculate the checksum of the region 0x1C000-0x1C02B, call it checksum(2).
Overwrite the current value with checksum(2).
0x23F40-0x23F41
This is a Checksum Reference to the previous reference checksum checksum.
Overwrite the current value with checksum(2).
0x23F9A-0x23F9B
This is a Region Checksum of the nearby reference checksums.
Calculate the checksum of the region 0x23F00-0x23F8B, call it checksum(3).
Overwrite the current value with checksum(3).
0x54700-0x54702
This is a Checksum Reference to the previous reference checksum checksum.
Overwrite the current value with checksum(3).
0x54712-0x54713
This is a Region Checksum of a dword which comprises of checksum(3) and zeroes.
Calculate the checksum of the region 0x54700-0x54703, call it checksum(4).
Overwrite the current value with checksum(4).