The game itself (the codez) are distributable under the GPL license. See link above.
The artwork is either GPL, or some Creative Commons license.
The name is not subject to copyright, but it can be subject to trademarks. AFAIK "Tremulous" has not been trademarked in this category (computer game), but it's a large world with many countries and anything's possible.
Anyway, I don't think any game dev company would be dumb enough to intentionally swipe the name of a well established open-source game and use it for their own game. I imagine there would be quite a backlash once the story hits the geek news and they'd come out as either stupid or evil (or both).