That shiny new symmetric gigabit fiber connection isn’t going to just sit there and waste itself, so bless Plex for trying to move beyond their customer base who serves huge amounts of pirated content.
Plex seems to have partnered up with Parsec and according to their FAQ, you can use can use any bluetooth or USB game controller that’s compatible with your device. For Chrome-based gaming in the browser, you can also use a keyboard. It appears that their frontend technology is based on RetroArch.
The service is not available for servers running on Linux, NAS devices, or NVIDIA SHIELD. This is probably due to their backend service Parsec, which does not run on Linux. None of the other high-quality, low-latency streaming protocols out there will do that, so either they re-invent a major piece of technology, or they ship without Linux support.