Use VGMt to extract iso image into Filesystem. Files are: .bnr (NGC banner), .bik (fmv), .txt (2 log files they seem), and the pair .gcs/.gcm which uses some custom/propietary compression format (BGIB) which is unknown yet/as for now.
GCM files seem to contain model data, especially with strings such as "GC Drawlist:".
GCS files are interesting though, as although they have what at first glance looks something like a WAV file header (has RIFF, WAVE, then fmt) the data is completely different, and seemingly has some of its own chunks for stuff like localisation and events, too.
VGMtoolbox is developed by snakemeat and officially found here on sourceforge. Yes, it hasn't been updated in a while, but the old GC ISO support should still work fine for this. I don't know about the additional features Manicsteiner added in their now-archived Github repo, but it looks like it's unrelated.
Seemingly a dead end at the moment unless further research is done.
Existing information on this file format is incredibly scarce. There's a decompressor for another version of it, however the data in Catwoman doesn't appear to be compressed. Tried it and it doesn't work on this game anyway. (There's also quite a few location mismatches and assumptions that fail in that tool too)