2 button remote transmitter and receiver in the house. Place a magnet ( the same one as the gate uses to sense origin in most cases) and a pair of outdoor reed switches ( the type you typically use on roller shutter doors is the most robust, though you will need to protect the wiring after the armoured tail) so that the one button of the remote is pushed by a magnet passing over the reed switch just before fully open, and the other is pushed by the reed switch just before closed.
That way the receiver has a contact close just before closed, and another just before open. Then you have in the house by the receiver a simple bistable with the 2 relay contacts simply setting it to one state or the other, preferably with a 12V battery and charger ( same type as the gate or a burglar alarm used) to keep state on power failure.
Otherwise, if you have an intercom wired to the gate there should be a spare pair in the cable, or you can draw in a new intercom cable with more pairs, so that you can simply use a pair of LED's in the house and a reed switch and magnet for fully closed and fully open indication.
