The advantage of DC motors is you can use a back-up battery in case of power failures. I have seen a few using a 12V sealed lead acid battery. The bigger installations use ac motors, which are more rugged and require less maintanance.
I've never seen a stepper motor used on a sliding gate. They normally just have 2 limit switches on each end which stop the motor. Correct placing of the limit switches allows for the momentum of the gate.