There's no point checking as aircraft regularly change gate, particularly at T5. PDX, as with all flights operated by a widebody, will almost certainly depart from either T5B or T5C, although there is a chance of a bus gate from T5A and a slimmer chance of an actual gate at T5A, although not impossible. You can use the Club lounge in T5B regardless, and walk back to T5A using the walkway if needed.

If you do want to check historical gates, the easiest way to do this is FlightRadar, which will show you the route of the flight and the location the aircraft pushed back from (in most cases where the transponder was active on pushback).