The default game mode is an important part of your server. It determines what game mode your players start in when joining your server.
Edit Server / Settings / Gamemode on your Server Dashboard. You're able to set this to any of the four Minecraft game modes. Once you've selected your desired game mode, click Save.
Additionally, if you would like to make sure every time a player joins, they start with the Default Gamemode, you can enable this in the "Force Gamemode" setting. This is located next to the Gamemode setting.