Did you follow the link and read?
In a nutshell:
Graphics in Qubes OS is handled either by dom0 (default) or in R4.1 optionally by a dedicated sys-gui. In any case, the programs running in your qubes will be using a software graphics driver and the resulting rendering will be shared to dom0/sys-gui via a shared buffer. This is so no program in any qube can access whatever is rendered in another qube.
Consequently, even if you get the Nvidia drivers to work in dom0 / sys-gui, the programs running inside your various qubes will not benefit from hardware acceleration. Your window decorations might render a bit faster, but I doubt you’ll recognize any benefits. Actually you might just burn countless hours trying to install proprietary drivers, thereby lower your security and in the end frustrated switch back to the integrated GPU because it simply works better in Qubes OS.
You could attempt GPU passthrough to a specific “gaming qube”, but it is my understanding that this is quite experimental and will require lots of troubleshooting. So unless you derive satisfaction from trying to understand how this (could) work in detail, you are probably better off dual booting … which comes with it’s own set of issues.