Hi, I wrote a long review about the cloud gaming service Geforce Now (by Nvidia) and I tried it on Qubes OS:
Here are the Qubes OS specific bits, to read after the review 
I wasn’t able to use a qube with a GPU, so this applies for all Qubes OS users
As stated in the review, without hardware acceleration the web browser will drop frames and Geforce will reduce the quality until it gets better. Playing in 720p instead of 1080p allows to cope better with the stream and keeping a higher quality.
Xbox One USB controller doesn’t work in usb passthrough, it’s not specific to geforce now though, the device auto detach itself… However the Playstation Dualsense (PS5 controller) works perfectly fine if you pass it to the qube with Geforce Now 
If you need a mouse, use an USB mouse and pass it to the qube with geforce now, the cursor will be captured by the browser focus and works pretty fine. Once you are done, the easiest method to get the cursor back to dom0 was to unplug the mouse and plug it back. 
There was no issue with the keyboard 
Globally, this can be a cheap and practical way to play some casual game, but this is not practical for anything serious 