US flights (except for 888, which goes via YVR) usually go out of 3, 4, 21 or very occasionally 19, due to the extra security checks required at the gate. Other than that, it's pretty much a random draw. Late night long-hauls mostly go out of 60-70, except the two LHR flights which almost always seem to go out of 1 and 2 (at least in my experience). As for regionals, I've had gates all over the airport, and even, gulp, the 520s once or twice.

Arrivals depend on where the aircraft is scheduled for next. So if your 77A on 889 is scheduled to turn around as 830 back to JFK, chances are you'll arrive at one of the gates for US bound flights listed above.