Literally every distro has seamless gaming.

To expand on this: Some distros want a little more setup than others for gaming, but it's generally a one-time thing and not particularly hard. Even my Debian Stable system plays games beautifully with a handful of Backports installed.

Is flatpak the way?

Steam can be installed natively or in a Flatpak. The latter can be a little fussy every once in a while, depending on your setup, but has the advantage of somewhat better sandboxing. (For better privacy protection, do your gaming on a dedicated machine or at least in a dedicated user account.)

What about Proton?

Proton is great. Steam comes with several versions, allowing you to choose the one best fitting any game that doesn't work with the latest/default version. Some users choose to add third-party Proton builds (like GE-Proton) to Steam alongside the standard ones, but it usually isn't necessary.