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
tremulousand 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.