haven't found an easy way to add gamemoderun to all steam launch options for all my games, so I just wrote this little script.
It's far from perfect, but works for me and maybe someone else has some use for it as well.
Some things worth to mention

• the script doesn't edit your original ~/.steam/steam/userdata/$YOURSTEAMID/config/localconfig.vdf
  but works with a copy in /tmp/
• you have to insert your proper ID (valid directory name in ~/.steam/steam/userdata/)
• it removes every LaunchOptions line first
• then inserts
  "LaunchOptions" "gamemoderun %command%"
  before every line with "Playtime"
• you have to copy the edited config manually back and make sure to close steam before.
• use at your own risk

GamemoderunSteamCommands.txt