You add a gate, where you add objects, in the third tab.
Here you insert the coordinates and the name of the map where you want the map to lead AND where the map will spawn you.
For example. If your central map name is Farrah Island, you'll want to add a gate at certain position, (wherever you want it to be) and then insert coordinates for where you want it to spawn you on. If you were connecting Farrah Island to say, Key Tunnel, you'd want the gate to have the map name KeyTunnel and then the coordinates for where you would spawn.
(It's best to pick this spot before making the gate, jotting down the coordinates and yaw, and then switching back to the other map to add the gate.)