If I'm not wrong, you can use these weapons provided you follow GPL rules (logical), so, mainly, if you modify weapons, you should also publish the sources of your modifications.
Also, you can't make a derivative of a GPL work based on ressources which are not GPL-compatible.
I think there is no problem if you use OA weapons as they are, while continuing your work on your Q3Rally mod.
From more distant concerns, the sad point is : the fact you're working on a closed source project prevents you from totally releasing your own work under opensource licenses.
It is better to build some work from totally opensource material, and to be able to give things back to the community, especially if you recognize some value to the opensource world.
But you're free

and I suppose building some q3rally mod from scratch is not really possible so it leaves you few choice ...