I wouldn't be surprised if Apple develops a library to help facilitate GPU usage, similar to how they developed Grand Central Dispatch to help developers utilize multicore CPUs more effectively.
Even better: what if Apple developed a whole language for GPU compute? They could eventually get other vendors to participate and make it an open standard. How about "Open Compute Language"? Nah, too verbose. How about "OpenCL"? ... =)
No - GCD is only about distributing workloads across CPU cores, and doesn't involve the GPU.
OpenCL uses a special programming model so you can't use it for general application code. It's good for doing repetitive operations on large arrays - e.g. Image or signal processing, or machine learning. OpenCL code will run in the CPU if there is no GPU or if the overhead of shipping the data to the GPU is too high.