Generally speaking, a game doesn’t execute on a GPU, it executes on the CPU & the rendering executes on the GPU. The GPU is specialized to execute graphics related calculations whereas the CPU is designed for more general purpose calculation. For games that use a GPU instead of the CPU for rendering, the CPU is still necessary for executing everything else. When you run have a game that renders using the CPU instead of the GPU, you have two main disadvantages:
- The CPU is doing more work than it would if the rendering calculations were being handed off to the GPU.
- The CPU isn’t as well suited to performing rendering calculations as a GPU (of equivalent vintage anyway; a modern CPU might be able emulate a significantly older GPU in some cases without any trouble).
Regarding parallel processes, some games do use parallelization to improve performance. But the gains aren’t linear, meaning past some point, adding additional parallel processes doesn’t improve performance. But it increases complexity, making the code harder to design, maintain & fix. Parallelization is a tool that is used when it makes sense, but it’s not a silver bullet solution for improving performance.