Your completely wrong on this. See my earlier comment.
The thing is, GCD is a framework that 'drives' OpenCL. It is nothing like CUDA. And GCD is accompanied by several other core technologies like LLVM and a C extension (closures) to make it much easier to implement.
I'v seen some experiment with it and a speedup of 20 to 25 times for some rendering applications like photo enhancements and movie effects is already shown.
And, thats a lot of PC's to buy.
And yes, OpenCL...