Is Tremulous trademarked? I think it is, meaning that you can't call anything tremulous without permission, and you're not likely to attain it until you have something working that looks good.
However, the source code is under the GPL and the assets are under some form of Creative Commons, which means you don't need permission to freely use those to make a game under certain conditions (most importantly, that if you redistribute a derivative work of the source code, you must publish that source code).