To set the default game mode for your Minecraft server using the server.properties file, follow these steps:

1. Stop your Minecraft server.
2. Open the server.properties
3. Look for the gamemode property and change its value to the game mode you want to set as default. The available game modes are survival, creative, adventure, and spectator.
4. Save and close the server.properties file.
5. Start your Minecraft server.

Your server will now use the specified game mode as the default game mode for new players who join.

Note: Changing the default game mode will not affect existing players’ game modes.