Let me just quickly fill you in on what I know. The vast majority of the GT score is missing, but there is a lot of interest in ripping the music from the DVDs, especially with how Funimation is encoding them (which seems to make it possible to rip the music without the voices/sfx). At this point, the best stuff you’ll find is on YouTube by typing in "Dragonball GT Unreleased." Most of what anyone would have is probably going to be a rip from YouTube anyway. In particular, a guy named "Colonel Silver" has a lot of really good stuff up that you should check out.
You may already be aware of this, but the OST for Dragonball: Path to Power has a bunch of GT BGM, and I know there’s a link to it in the download links section of the forum. This constitutes the only official GT BGM release at present.
To that end, hopefully someone will upload some high-quality rips sometime soon.
Edit: One other thing – some GT BGM is in the soundtrack to Dragonball Z Sparking! Neo, the Japanese soundtrack to one of the Budokai Tenkaichi games.