Depends on your fare bucket as a Gold member. Some fares don’t qualify for an upgrade to premium class until 24-72 hours before boarding (depending on availability). If you’re seeing a charge when selecting a premium cabin seat, then that’s why.
Sorry just to clarify: yes, the fare bucket of my already booked travel is high enough
to already auto-upgrade at the time of booking to premium IF I were gold now. Let’s say it’s all “S” class; so if I use this Gold milestone today, can I go into all of my future trips - even those further than two weeks out - and upgrade those also for free?