how can i change to a compatible pixel format? by editng xorg.conf?

I haven't used xorg but probably that conf file has the info on pixel depth.

> I think that is what causes glXIsDirect to return false (and triggers
> that message in GEM).

what is 'glXIsDirect' ? where did you see that? which message in GEM ?

forgive me my slow understanding and thanks a lot for your help.

glXIsDirect checks that the GL can be drawn directly to the screen and not be touched by the x11 server.

http://www.csee.umbc.edu/help/C++/opengl/man_pages/html/glx/xisdirect.html