Two weeks ago I tried variable DC on a modern gateman, lasted about 30 minutes before it melted some insulation in the coil and shorted out. I had it adjusted as low as I could and still get the gateman to come out reliably. It had a duty cycle on an oval under the Christmas tree of about 10seconds on, 10 seconds off. So I wouldn't recommend it.
That said, 20 years ago I ran the old solenoid Lionel crossing gates on DC with no issues. Perhaps there is something different in how the new v. old solenoids were manufactured.
