We have one of those where I work and it is as you describe. The gate is opened with an electric motor driving a reduction gear that pulls a chain opening the gate. I have never looked at it in detail but since I'm leaving anyway, I'll take a closer look on my way out. The shroud is locked so I wont be able to see the drive itself. If those details are important to you let me know and I can ask one of the maintenance guys about it. They're damned good at what they do.

There is an induction wire on one side and a key-card reader on the other, allowing easy exit and keyed entry. The thing is programmable but that again, comes under the control of maintenance.

Have you looked at GTO "Gates That Open"? Never used them but formerly a customer of mine.
http://www.gtoinc.com/

I think this is their DIY product:
http://www.mightymule.com/

Okay, here's what I could see.

After the reduction gear, there is a sprocket that rotates on the same axis as the motor.

Perpendicular to that sprocket is the chain, which is laying over the sprocket.

On either side of the sprocket are two roller-tensioners that keep the chain in contact with the sprocket and are adjustable to remove slack from the chain, minimizing the catenary.

The chain is attached low on the frame at either end of the gate.

When the switch is closed, the motor rotates to open the gate. After a short amount of time, and if an IR safety system doesn't detect anything in the way. the motor polarity seems to be reversed and the sprocket rotates in the opposite direction.

There is also a safety switch that works on pressure, preventing the gate from crushing a car or pedestrian who eludes the IR beam.

Hope that makes sense.

My challenge now is suspending the gate on rollers keeping it aligned top and bottom at opposite ends. I guess this is more a problem of a moving target due to the soil shifting the fence around.
Three years ago I installed the 6 foot fence and last fall I pulled up the west 70 foot side to set the post deeper hoping it will stay straight this time.
In 70 feet the top was meandering 6 inches. String line when installed.
Pisses me off.

Rolling the gate on a track will line the bottom up. I figure I'll have to make a large "V" at the top to guide the gate closed.