GN could pick any Linux distro and get metrics but they'll want to be consistent and pick what will be most relevant.

For most people SteamOS will be the pick when moving away from windows pretty much entirely because it's "Valve's" OS, who are a known entity compared to "whoever makes this Bazzite thing" (or whatever other distro). This will give them more confidence to switch as they are familiar with/trust Valve to some extent and know that they're going to make sure their os works and can play games as well as can be expected on Linux. Obviously we know you can do it on other distros but switching your OS is a big change and people will want more assurance.

GN could cover performance on other distros but I think that would probably just be noise to people who are on the fence/not that interested compared to "here's how things work on Valve's SteamOS". (Also probably just provides a clear starting point and direction for GN)