Gold does not give lounge access. On AA anyone can choose seats for free, even without status, though they are usually in the rear of the plane. With Gold, you can choose main cabin extra seats starting 24 hours before the flight. I don't know what that translates to on QR, though.
Gold doesn't, but J (Business) does get lounge access
Seating depends on the airline. AA does not have business class seating in the rear, and on AA flights, business class can pick seats