Because the gate has a property field of it's own so it's not hard at all?
Just create a gate, then go into the object tab and make sure "gate" is marked, not object or water.
Once you see the white square box around the gate, and that the object tab shows gate is selected. Go into the objects tab in the map maker, and you will be able to fill in the name battlefield in the mapname box.
It's not hard, just play around with it if these instructions weren't enough. Just make sure you save the map after you added the text for the gate.