Like Timbo said, the licensing of Tremulous is perfectly clear and well described for both code and content.
The major complain of FSF, GNU people and BSD people about Creative Commons license is their lack of precision on certain details, and the usage of the generic expression "CC license".
Never say something is under a "CC license", because CC include a dozen of licenses with very different levels of freedom.
Most of the Tremulous content is not under "a CC license" but under the
CC-BY-SA license. And this is the CC license that is most similar to the GPL.
The licensing of Tremulous content is an incredible gift to the community of game developers. Games with this level of quality and freedom could counted on the fingers of one hand !