I’m pretty sure GPU passthrough does work in Qubes HVMs, although I haven’t tried it myself.
Here are three quick and recent tutorials I found including one with a newer VirtualGL approach that offloads work instead of passing the entire card.
Does this fix the Code 14 issue with NVIDIA cards? That is why I had to switch to KVM back in 2016, as KVM has support for bypassing NVIDIA's "bug" which prevents using consumer cards in a virtual environment. I have been away from Qubes for 7 years now so I'd hope some form of improvement has been made.
https://neowutran.ovh/qubes/articles/gaming_windows_hvm.html
https://forum.qubes-os.org/t/nvidia-gpu-passthrough-into-lin...
https://forum.qubes-os.org/t/seamless-gpu-passthrough-on-qub...
Yes, the passthrough is probably a huge avenue for attacks. Possibly VirtualGL too, I know less about that.