I tested it with gamemode's own 'fake' game and it seems to work just fine, when I run 'gamemode-simulate-game' it runs just fine for 10s without complaining and then exits.
Can you give some information about your system? Is it on stable or testing or unstable? How are you launching the game? (standalone installer, steam, etc). I don't have access to rise of tomb raider but it seems that gamemode is doing what it's supposed to do. Also check whether gamemoded is running with "ps aux | grep gamemode". -Jonathan
