A reference to the game mode was added in May 31, 2016 update.
The mode is referred to in the console error message when
gamemode command is given a wrong value:
Missing gamemode argument. Valid arguments are: Valids arguments are: 'casual', 'competitive', 'arsrace', 'deathmatch', 'custom', 'demolition', 'guardian', 'tilegen'
- Swarm TileGen on Valve Developer Wiki