And would passing through a GPU via devices tab in VM settings actually be enough?
All depends on what game you are referring to, and what it requires processor and RAM wise, and the speed it requires to pass information through.
I game on Linux using Steam.
yes it is possible to game on linux. I game on steam on a barebones debian system. cs:go and dota2 are some supported games. Some games are playable with play onlinux scripts using wine. But in Qubes-os vms do not have 3d graphics rendering, so you can't really game in qubes. It might be possible to pasthrough your gpu to a vm, but this is not officially supported and not too many people have been sucessful.
On another note and imo, playing online games defeats any security you have every time. There is no such thing as security on your machine if gaming online. Especially when your gpu gets infected. Gaming online is where most cyber criminals get started. They are the most hacked networks. It is where Edward Snowden learned to hack people, Sabu, Kim Dotcom was notorious in q2. Online gaming communities, where only hackers are left playing, are as dangerous as irc or the dark web. They don't just want to ragequite you ddos the game offline and steal your credit cards, they want to destroy your hardware. Especially if you are not one of them (Russian)
Some argue if you have a separate gpu that is isolated from dom0 and only loads in a vm then maybe this would be safe. But I still have my doubts.
And how would it be performance-wise overall?
Especially with Windows 8+ cause theres no official Qubes Tools Support there.
I've had no success passing through a pci device to a windows hvm. But it give it a shot and try.