None. Send patches. Though, there are ideas floating around and supposedly someone working on something but its vaporware as of now.

A plan 9 interface to a GPU would have to be as general as possible by excluding API specific interfaces. This way you have a GPGPU interface which you build applications/api's on top of.

edit: Here's a very back of napkin idea that was lightly discussed recently, https://marc.info/?l=9fans&m=162965057312200&w=2