CPU and GPU can't be used interchangeably. They are specifically designed to be good at some kinds of computations and bad at others. (because building one single processing unit that's good at everything is not possible)
Think about it: if CPU and GPU could do the same work, why would GPUs even exist? Why not perform all computations on CPU?
GPUs are a thing because CPUs are bad at processing graphics. Originally they weren't capable of performing general purpose computations at all, it's a relatively new idea.
So to recap, in general you can't offload some work from CPU to GPU or the other way. A program must be specifically designed to be able to do this. Basically, the algorithm must be implemented separately for CPU and GPU, because they work in totally different manner (CPUs are SISD, GPUs are SIMD).
Some programs can do that, notably some video processing software. There are also games which will let you choose whether physics are calculated by CPU or using PhysX hardware on the graphics card. Unless explicitly permitted by the software, it's not possible to select which unit will perform computations.