[1] https://groups.google.com/forum/#!topic/qubes-devel/MfHy2jmX...
But PCI passthrough for gaming isn't a great fit for the Qubes security model: it requires trusting that the Windows guest cannot compromise the GPU you loan it, which makes it a much bigger risk than an ordinary AppVM.
Yes, a dedicated machine for gaming would certainly be better. But I play only irregularly and games like Divinity: Original Sin, Pillars of Eternity etc. which I kind of trust to don't hijack the GPU.