As far as I know, the soundtrack that comes with this GOG release is the fan made one, because Thief never had an official soundtrack. Don't get me wrong, I love to see it here along with the game, yet since GOG sells this for money, how is this for the guys who did the unnoficial ost?
I was reading the TTLG forums and it does look like this is the fan made one you are mentioning. The author of that one commented: "no, GOG never asked for permission to use my soundtrack. I don't care for a royalty for stitching together what has already been there, but a side note from them to honor my work would have been nice."