Buffer management is in kernel space, but that's about all that happens in kernel space GPU-wise. Indeed, most interesting things happen in user space.

Most hardware doesn't have it's own MMU like most GPUs have had for over a decade. That lets GPUs run most of the data plane in user mode right next to the code calling into OpenGL/CUDA/Vulkan/etc.

It is much safer to put complex software into userspace. If there is an error, you won't get kernel panic.