Unfortunately you can't fork a coin away from gpu, unless you make the CPU miner closed source.. GPUS are re-programmable. you can fork away from an asic as its stuck doing one algo.
just sayin.
You can't fork away from a GPU miner but you can make a GPU miner useless... GPU's dominate CPUs by its processing power as it contains lots of cores vs a CPU that only has an average of 4 cores... But what a CPU has that a GPU doesn't is a high memory. If you make a algo memory intensive then a GPU won't perform better that a CPU. And that's how you make a "CPU mineable coin".