Hm, GBS isn't like you people think.
MP3, for example, has like, details of every short part in the file - everything about the music is in the file.
However, for GBS (and VGM, NSF, SPC, GYM, and other game audio files), the file contains, like, notes that WinAmp can play. This is the way they were in the original game, and is the reason why the filesize is so extremely small (48kb for an entire soundtrack).
However, this also means that you can't just 'convert' the songs (although you can easily record them and save as MP3). Also, the songs don't have an ID3 tag or whatever - in fact, all songs are in one file, and you can only identify them by their number.
It is also highly likely that the songs just loop indefinitely, so you'll have to cut and fade out at some point if you want to make an MP3 out of it.