Scheduled vs. Actual Departure Times

Now, I know that the FAQ says that the departure and arrival times given are the wheels up/wheels down times. But are the “proposed departure” times the same, or are they gate times? The reason I ask this is because the different between the proposed and actual, as given on the tracking page, is usually about ten to fifteen minutes, which is a pretty average taxi time. So is the proposed time gate or wheels up?