The only time I've ever heard of "general admission" with regards to Roundabout is if you're a subscriber and have to exchange your tix and you exchange them too late, then you can get gen admin tix for the night you want to go...I don't think it's a 'regular' thing that this (or other theatres) do...
Ok, from Roundabout's subscriber info:
Exchanging subscribers receive the best available seating at the time the exchange is processed. If a performance is sold out, you may receive a General Admission ("GA") Pass. Subscribers holding GAs will be seated by the House Staff just prior to curtain and may be asked to take different seats at intermission.
You will not be interrupted during the performance.Roundabout Subscriber Guide