Maybe that's where they run the HDCP/DRM stuff? That's the most obvious application of a secure enclave on a GPU that I can think of, and would also explain why they won't (or can't) open it up.
Isn't HDCP/DRM implemented in ASIC hardware? There is nothing programmable there.
It's probably implemented mainly on the Platform Security Processor.
GPU is itself a standalone computer now, so are CPUs, might as well use the same thing for housekeeping tasks like controlling voltages and clocks.