I've been planning on doing exactly this, but haven't had the time to figure out the best way of going about it.

Is there a good guide/write up you'd recommend? (I already have a Windows 10 install on a separate SSD, but I rarely boot it because I can't stand the OS)

Can theoretically game on Linux and windows at same time with just a single gpu (2080 ti), mainly use it cause I only have single gpu and I don't want to loose hardware acceleration on Linux, the nixos module both applies a the vgpu unlocker for consumer graphic cards (so u can split a gpu in multiple virtual gpus) https://github.com/DualCoder/vgpu_unlock

And merges it with the GeForce drivers so the host gpu does not stop having display output

And I just passthrough the vgpu and Xbox controllers to the vm with qemu and my main windows nvme disk (windows naturally just works inside and outside a vm for me, dual boot)

When I moved to NixOS I just did a normal NixOS install and applied the same parameters to kernel boot as before and passed the GPU through.

I was using looking-glass at first, but my computer is too slow and crappy (memory bandwidth seems to choke) to run 144fps in overwatch.

You just pass the entire block device into the VM if you wanna keep your setup, or pass it through with as PCIE if it's an NVMe device.

I thought it'd be hard, but you'll be up and running in a day

https://www.youtube.com/watch?v=voXc1nCD4IA

This video is a pretty good demonstration of how neck-and-neck the performance is these days. Frankly, I feel like it stutters less in Linux, but I might just be imagining things.