This is how I set up my levels to be loaded in GM mode.

Start project>Addon>New Level>Target for GM

In level data menu make sure GM is True
In Project Settings make sure everything is set to GM
Publish Project should say *addon* *GM* >Publish local.

In game, Prepare Campaign>New Campaign>Make sure Addon is checked>Create

I'm still new to modding so there maybe things I've missed. But I've been successful getting multiple maps in this way.