If I recall correctly, there was a GPU miner for primecoin that somebody claimed to be developing, and took donations for it, but never delivered.

As far as I know there still is no GPU miner for primecoin, mostly because the CPU is still needed to do certain things efficiently, but the communication time between the CPU and GPU killed any performance gains from the GPU running any part of the code. Or it was something to that effect, AFAIK.

If it would be possible to make a GPU miner I think that would be better long-term for this coin. CPU coins get dominated by botnets, drowning out potential profits from people running it legitimately, since botnet operators don't care about electricity costs. That kind of kills a lot of potential enthusiasm for it. There's only so many people in the world willing to lose money calculating prime numbers for fun.