How to change the default game mode

The default game mode is what a player is put into the first time they join. It does not change anyone who has already played.

Change it

1. Open the Settings tab.
2. Set the game mode you want.
3. Save, then Restart.

It only applies to new players

This is the part that confuses people. Once a player has joined, their game mode is saved with their player data and this setting no longer touches them. Changing the default to Creative will not put your existing players into Creative.

To change someone who has already played, use the command instead: see How to change a player's game mode. To move everyone at once:

gamemode creative @a

Force it for everyone, every join

There is a separate force-gamemode option in server.properties. With it on, every player is put back into the default game mode each time they log in, whatever they were in when they left.

1. Open the Files tab and edit server.properties.
2. Set force-gamemode=true.
3. Save and Restart.

This is how hub and minigame servers keep players in Adventure. It also means an operator who switches to Creative loses it on their next login, which surprises people.