I am on a fully libre kernel (Linux-libre, to be precise) and system for years and get 60 FPS for the games I care about.
The key to everything is the hardware. You need to research and check whether a libre Linux officially supports it. Make sure all the hardware you have is supported.
If you have a hardware that's not supported (or not supported well enough) by a libre kernel, the only reasonable thing to do is to replace it. Don't try to “force” a hardware to work (e.g. by searching the Internet for weird workarounds) if you know it's not well-supported by your kernel or software anyway; this is just going to be a huge waste of time, will seriously frustrate you and even if you
somehow manage to get it to work, the result will disappoint. I did the exact mistake as you in my early days of Linux: I had banged my head for months trying to get an old sound board to work on Linux, searching dozens of forums with no success until I finally realized there just
isn't any Linux driver or anything I could "install" to make it work. It's just
not supported because the driver I need
doesn't exist. Keep in mind a lot of the hardware isn't manufactured with Linux in mind.
So if a hardware is not supported on Linux, accept it,
move on and find a replacement.
As far as graphics go, NVIDIA hardware has to fly out of the window, as the libre driver performs worse and it's probably not worth your effort. Why is that? Because the libre driver (Nouveau) developers are forced to reverse-engineer everything of NVIDIA hardware because NVIDIA hates free software and doesn't support the FOSS community at all. It's no surprise the libre driver underperforms. Thankfully, NVIDIA does NOT have a monopoly on computer graphics.
What you want to use instead is some other graphics hardware for which you know you can get first-class libre drivers for (i.e. definitely not NVIDIA). I personally use Intel hardware btw, it's well-supported on Linux.
My key point here is: You absolutely can have a functional performant gaming system even if you go 100% FOSS mode. High-performance FOSS gaming is possible, and it's real. But it requires research and patience to set up a good system first.
Note your gaming system might probably not be the highest end ever, since the super duper graphics hardware might be locked behind proprietary drivers, unfortunately. But chances are, you probably don't need the latest and greatest hardware anyway; what's much more important that it fits well into your overall system and is compatible with everything else. Maybe one day the FOSS movement will be strong enough to even “infect” hardware manufacturing (there's still so many trade secrets that drive libre driver authors mad), but we're still far away from that.
And finally, the biggest limitation that you can't work around in FOSS gaming, obviously is still the games themselves. Games are still very dominated by proprietary software, that's the harsh truth, but who knows what the future will bring?
Anyway, it can still be a fun excercise to build up a gaming system that is built on 100% FOSS, and I hope I could help you a bit.