For Steam users, the default is usually C:\Program Files (x86)\Steam\steamapps\common\GalGun Double Peace . You can find this easily by right-clicking the game in your Steam Library and selecting Browse local files .
Useful for viewing and exporting textures from .upk files. File: Gal.Gun.Double.Peace.zip ...
Open the .zip file and extract its contents into the appropriate subfolder (often \GG2Game\CookedPC\ ). For Steam users, the default is usually C:\Program