Right now sys-gui-gpu can be used only for security reasons to remove the graphics from dom0:
Furthermore, while in theory dom0 is isolated from the outside world, some graphical devices (e.g. displays connected via HDMI or DVI) offer two-way communication, which threatens this isolation and makes it harder to maintain. If a malicious device (rather than the user’s trusted monitor) were to be connected to one of these ports, it could inject data that could be processed inside of dom0. As long as graphical devices are in dom0, they also cannot be safely proxied to other domains. This is because the various solutions to multiplexing access to the GPU at the GPU/driver level (which would expose the “full” GPU to a VM) are orders of magnitude more complex than running display drivers in just one place. We consider this added complexity too risky to put it in dom0. Errors in the drivers could expose dom0 to an attack, and attacks on dom0 are the biggest threat to the Qubes security model.
There is a project to provide GPU acceleration to virtual machines, but it’s a WIP for now:
It seems that your sys-gui-gpu wasn’t configured correctly, maybe the salt formula is bugged.