GPU only affects encoding if a) the software is specifically designed to use the GPU (i.e. designed to work with nVidia's CUDA cores), or b) there's a particular encoding feature used on the GPU (but again, only if the software can access it).
That being said, GPU-based encoding offloads some of the work onto the GPU, but you're still going to rely quite a bit on the CPU. You'd probably have to look into specific benchmarks to see how much GPU hardware can help, but again it's going to depend on the combination of CPU, GPU, & software that you're using. For example, using an application that offloads the work specifically onto CUDA cores will probably see zero benefit if you have an AMD GPU in your system, because CUDA cores are...