Well from the looks of it, it appears to be some form of Lex Encoding usually used for Source Code and Archival Compression with the gp2 extension which some how adds multiple LZ compression methods on one file, plus has to be an archival file much like ZIP or 7z. but it looks like it more of a generic format used by the Nintendo Dev kit system, apparently a format that would also have another purpose,
Making sure no one can deccompress or configure the files on rom hacking terms, as a build on protection to it, remember, the Ani-Piracy issue still exists in these roms and some how they knew how to make it more diffcult to read the files or edit them.