Yeah, I just finished emerging Tremulous a while ago, but I'm not sure how Portage handles it. If you go to tremulous.net, you can download a standalone installer, which does NOT install Quake 3, which may be a faster install. There's no tremulous binary in /usr/bin after I emerged it, which is how it usually goes, so I'll have to find out where the heck it put the thing/how to start it. (emerging slocate for just that)
Edit: The game doesn't launch because you must also emerge quake3-bin. However, the version in portage is not the latest, as Tremulous 1.1.0 has a standalone installer that you can download at tremulous.net in the files section. I think I'm gonna unmerge the quake3 stuff and then installer Tremulous manually.