For a while I daily drove a Purism Librem 14 with Debian's fully free kernel, and installed as few non-free packages as possible, including firmware blobs (which I didn't install any of until I decided I needed Bluetooth). My experience with gaming was generally fine.

With linux-libre you really have to buy your hardware specifically with support in mind. You're limited to Intel and non-bleeding-edge AMD graphics cards, a very small range of wifi cards, and no Bluetooth. Otherwise, video games should work as well as they would on any other computers with the same specs. Especially if you're also limiting yourself to games with free engines - I'm not aware of a single libre game that demands more than a modern Intel integrated graphics card can provide, even on high settings.