That's not something for Cosmology@Home to fix.
In the least, (register and) post about it on the BOINC forums, in the GPUs forum. You may want to try to old stickied threads there telling how to get the GPUs to be recognized on older versions of your distro.
But in the end it always boils down to one thing: drivers.
Either the drivers are not right for your distro, or they load too slow and BOINC too fast (easily solved by waiting 10 minutes for the OS to completely start, X to start and then (re)start BOINC.)