https://wiki.xenproject.org/wiki/Understanding_the_Virtualization_Spectrum
For gaming qube you most likely use HVM virtualization, when most secure is PVH.
For GPU passthrough you should use HVM, and can’t use PVH.
QubesOS by default mask your mouse pointer, but in order to play many 3D games you should fully expose it to gaming qube.
Infected mouse may infect gaming qube and vice versa, and spyware risk that let programs running within qube to direct access to mouse data.
How it works:
How to use USB devices — Qubes OS Documentation (for gaming you should change this behavior)
Hope you have few billions to open new brand semiconductor factory, and keep it out of any state influence.