Does anyone know what type of files these are, and whether they can be renamed and extracted? Or is .gog a proprietary file type meant to prevent this very kind of thing from happening? I'm also sort of curious as to how the application recognizes these files and the associated .inst files, just from a purely technical standpoint (although I should point out that I'm not a computer science major, just a curious layman).

These are only the cd images of the games, to get the in-game soundtrack :)
Try renaming them .bin / .cue if you want to test. It may be useful to rip the soundtrack in FLAC for example.

Yeah, listen to DukeNico :P
.inst = .cue
.gog = .bin

Ah, this is exactly what I wanted to know. Major props to both of you! :)

Quite a clever way to solve the issue of missing cd-audio in DOSBox IMO (it's never worked for me, anyway). Those twats at Steam couldn't figure this out (example: Quake). The audio files also appear to be encoded lossless (no MP3/OGG crap), so major props to the people who put it together the way they did.