The programmer decides, which tasks will be performed on CPU, and which - on GPU. GPU is a full capability processor that runs its own program, so in theory there is no limit on what it can do - the only limit is time. The programmer must write a program for GPU as well as for CPU. Programs for GPU are called "shaders".

If we talk about games, the GPU usually does at least 3 jobs: convert all 3D models to screen-relative coordinates, sample colors from texture images, and calculate light and shadow. However, more tasks get moved on GPU with time: modern games use it for simple physics simulation (hair, clothes, fire, smoke, rain, grass), and for post-processing (color correction, blur).

GPU can also be used in tasks unrelated to graphics. Many scientific physics simulators and machine learning tools have an option to run on GPU.