This is a well known problem - I had it with my old (matrox) gfx card (yes, it can be resolved!)
It can be found on the forum by searching for this
Sys_Error: GLimp_Init() - could not load OpenGL subsystem
The topic is named the same way and it is here:
http://tremulous.net/phpBB2/viewtopic.php?t=831There are several solutions for the problem but in my case typing this:
export LD_PRELOAD=/usr/lib/libGL.so.1
in console before running the game (yes... you have to do this everytime after rebooting the system

) solved the problem.
But to make it easier you can make yourself a script:
export LD_PRELOAD=/usr/lib/libGL.so.1
tremulous
and run the game by running the script (I put the script on the desktop)
But first make sure you have the latest drivers for your card.
And, of course, I don't know if it will resolve the problem - I only said that it helped in my case. But if you are sure that you have a gfx card with 3D acceleration and you have OpenGL drivers for your card installed then it should help.