FlightAware uses runway arrival and departure times once they are filed, whereas the United app will show scheduled until the plane leaves (and uses gate times if the flight is delayed). Not sure whether it then uses gate or runway times for "actual"; can't say I've ever relied on the app since FlightStats shows faster-updated filings of both runway and gate times.

The flight record does keep four times though: gate/runway arrival/departure, and most apps usually choose semi-arbitrarily to use or the other. Things get messy when (to use seasonal regional experience) a flight gets a tentative arrival slot at SFO and a 3-hour delay posts for runway arrival without gate arrival updating. This generally indicates ops doesn't want to publish the delay (it won't show in the United app), and to be fair it sometimes changes.