To be pedantic, GPUs are no better than CPUs at multitasking. GPUs are best at performing one task on a moderately large set of floating-point data (SIMD).

The Tesla vehicles use cameras, ultrasonics and radar. So they are processing graphical data (a video stream) and signal data. That's exactly the kind of task that GPUs are good at.

However, this only gets you as far as perceiving the world. The vehicles also have to decide which action to take -- following the traffic laws of the jurisdiction they are in -- which is exactly the kind of branchy imperative task that CPUs are good at and GPUs are terrible at.