Set the default game mode new players get when they first join the world
/gamemode changes the mode of a player who is already in the game right now
whereas /defaultgamemode sets the mode that a new player will get on their very first visit to the world. Players who have already joined are not affected.
The examples below come out of the same generator as the form above, so the commands and explanations always match what the tool produces. Copy one and adjust it from there
This only affects players joining the world for the first time — it does not change anyone already playing
/defaultgamemode survivalWhat it does Will set the world's default game mode to Survival. Players entering this world for the first time get this mode, while returning players keep their own current mode.
Adventure maps set this so new arrivals cannot break the scenery
/defaultgamemode adventureWhat it does Will set the world's default game mode to Adventure. Players entering this world for the first time get this mode, while returning players keep their own current mode.
Suits a build-only server — new players arrive with everything available straight away
/defaultgamemode creativeWhat it does Will set the world's default game mode to Creative. Players entering this world for the first time get this mode, while returning players keep their own current mode.
This is the first example above, run through every edition and version this tool supports. Versions that produce the same result share a row
Every version this tool supports produces the same result, so a command you copy works without version-specific edits
/defaultgamemode survival