Is there some kind of tool to decrypt these crypted gibberish letters and number files?
There's some research going on about those files, actually. The files are encrypted, and when decrypted there is a ZLib stream to decompress.
We're able to pull files out of Gal☆Gun Returns upon file load, but being able to open, change, and re-insert the files directly is a different story altogether. Each file seems to require a different hash to decrypt with, but from there we're able to decompress the ZLib stream inside the files and get what's needed, although there is the matter of compressing and re-encrypting.This tool
allows someone to decrypt the files manually, but a hash is required.