Head on over here
. This is the SierraGamers bulletin board; go down to the "Music/Soundtracks" topic and you'll find a "Gabriel Knight Mysteries soundtrack" topic within. This is, of course, the audio CD that came with the GK Mysteries limited edition, which I happen to have bought...twice
You can also find the QfG5 (audio CD) soundtrack, some Inca music (never played it, so what do I know
), the Sierra Soundtrack Collection and Shivers II: Harvest of Souls (audioCD).