Yes. GST refund counter is airside on Departures level in International T1. It's just after immigration clearance. IIRC the Customs place for verification of bulky goods that are checked is on the arrival level but this might have moved in the last year or so.
Having your info on the app will make the time waiting much much shorter. Staff prioritise app people with the QR code over others.
This is a great improvement over the old system. Truth be told I always thought that the inefficiency was to discourage getting the refund, but maybe not. One other question, getting a new US passport, will I be able to use smart gate? Anythi n g I need to do before I leave Us..Other than the ETA.