As a bonus, it also gives an indication that the gate is "in transit" when it is moving open or closed.

The heart of this system is a  surplus "Banner" OPBT3 weatherproof photoelectric unit.
These things can detect an object about 12" away and are used in industrial applications to perhaps count objects passing by on a conveyor belt. These can be found for $25.
Banner photoelecric products
 

 The Banner unit is mounted so that it "sees" the vertical bars of the gate as the gate moves and then it sees a blank plate when the gate is fully closed. It relays it's signals to a garage on the hill in the background.
 

Here's another shot of the banner sensor mounted on a PVC fitting

 

This is the "old way" using a "door sensor" from a burglar alarm and a magnet to detect  when the gate was closed. It worked for about a year and then the magnetic switch failed.
The magnetic switches are delicate and cannot tolerate much electrical current even though the detection circuit only used a few milliamps

Here's a shot of the repurposed alarm sensor comprised of a tiny magnetic reed switch inside a metal rod.
There's a critical distance between the sensor and the magnet

Control Module
All from surplus or repurposed electronics parts
(L-R) relay with LED dropping resistor, solid state relay, power supply
 

Remote unit in one house
Green power light says everything is OK at the gate
Pushbutton opens and closes the gate

Remote unit in other house

Schematic Diagram

Old trailer stop light contains the ballast lamp
The wood box has holes on top and bottom even though the LED Christmas tree bulb inside generates very little heat.

How it works:

The heart of the system is the surplus weatherproof photo sensor. This one was probably used to detect objects on a conveyor belt. With the photo sensor mounted to the gate post, the gate position is sent to a remote location over a buried telephone cable. When the gate is commanded closed, the residents are able to positively know that the gate is closed by watching  the position indicator LEDs. As the gate moves, the lights  flash to give an indication that the gate is indeed moving. The bars of the gate alternately cause the photo sensor to "see something".

The photo switch is able to conduct only a very small amount of current when it is detecting an object, so we had to employ a circuit that only used a few milliamps of current. A Solid State Relay (SSR) uses 1-2 mA and was readily available on the surplus market ($6). The idea was to have the SSR actuate a larger dry contact relay where we could direct voltage to two LEDs that would indicate the position of the gate.

The SSR does not like inductive loads like relays so we also placed an LED "ballast light"  in parallel with the relay. This makes the SSR happy and indicates the gate is closed to the entire empty garage. The rest was easy. Using the single pole-double throw contacts on the relay, we send voltage to the remote LEDs that correspond to the position of the gate. A common dropping resistor of 1000 ohms was used to limit the current to the LEDs to about 12 mA at 12 volts.

Position control board mounted to wall of garage before protective cover is installed
Note this earlier version where the ballast lamp was a really nice "1950s" green indicator lamp. Unfortunately it ran hot and went through several bulbs before we decided to switch to an LED in a small Christmas tree bulb base.

Surplus telecom "66 Block" usedfor multiple systems interconnect around the tree farm.
There's a diagram to this mess above .Ma bell would not be proud of me in the wiring block dept. Western Electric who built a lot of this equipment was pretty smart. The "66" Block pictured here works very well to interconnect various cables around the tree farm even though it was used for offices and large phone systems for years.
It

Tree Farm Home