Found 20 thsd. answers for 'steam os how to boot on gamemode'.
https://steamcommunity.com/groups/steamuniverse/discussions/1/648814395987401860/
https://steamcommunity.com/groups/steamuniverse/discussions/1/616189106476217919/
https://www.youtube.com/watch?v=vNWb-kO0RyQ
https://www.youtube.com/watch?v=8pRwtzrolKM
https://www.youtube.com/watch?v=iiO8ac3_V9c
https://www.pcgamer.com/how-to-dual-boot-steamos-and-windows/
https://www.pcgamer.com/download-steamos/
http://osxdaily.com/2015/06/07/stop-steam-opening-automatically-mac/
https://github.com/FeralInteractive/gamemode
https://github.com/ValveSoftware/SteamOS/wiki/Troubleshooting