You cannot decrypt a file from GodMode9. There isn't the option in the menu, or elsewhere. The last thing you can do with gm9 before going on the computer is extracting the base_LZ.bin.
@Xdqwerty, Osu! Tatakae! Ouendan! is the Japanese version of the game, different settings/characters/songs but otherwise identical mechanics. I played that before I knew about Elite Beat Agents lol. Both fantastic games https://en.wikipedia.org/wiki/Osu!_Tatakae!_Ouendan