Is GPU memory wiped before it's handed to a new process, and is a process prevented from accessing GPU memory of a different process? (assuming low level APIs like Vulkan)
In theory you could have some sort of complex per-process scrambling system to avoid leaking information, but I think implementations actually just zero the memory.
GPU drivers on different operating systems can be more or less buggy; Windows and Linux generally seem to do the right thing, but MacOS is a bit more haphazard.
You can't just peek another process' GPU memory thru UMD app, either. Per-process virtual memory mechanisms similar to CPUs are also present in GPUs, which is the whole reason that resources are explicitly imported/exported across APIs via special API calls.