Especially if you’re not gonna play stuff that the anticheat locks you out from, the experience is great. As other commenters have said, ProtonDB.com has resources for how well games on steam run under Proton / On Linux.
Although, I would recommend Nobara Linux over Chimera OS due to a lack of experience with Proton and other gaming-related tools (as in, Chimera developers’ lack of experience). Nobara Linux comes from the same developer as Proton-GE (GloriousEggroll). Proton is the tool that Valve developed to run Windows games pretty much seemlessly, and Proton-GE adds extra features and patches on-top of it that can help support more games or get the slightest extra bit of performance out of Proton. Nobara Linux extends this concept to the entire OS, with a stable Fedora base that gets a major update every ~6 months.
Nobara also consitently outperforms other Linux Distributions and even Windows regularly.
(This doesn’t mean that you don’t get updates for 6 months, just that major releases, e.g from 39 to 40 happen every ~6 months)
The spellchecker by default does (as part of some initiative regarding improving spellcheck or whatever). There is a setting to disable it and another setting to disable telemetry. Disabling both of these effectively makes the keyboard offline apart from things like Stickers and Emojikitchen.
You can also block internet access to GBoard and it’ll work just fine.
If you don’t hard-depend on GBoard, you may wanna look into Florisboard and Helium314’s OpenBoard Fork