I'm not a member of GAP and have been offered the ability to participate in several beta tests. Beta testing is simply an option as a result of being an Underground member who indicates that they are interested in the beta test process, as the pic of the form indicates.
GAP is separate. I got an invite for this by mail once, but had to RSVP by a specific time and didn't. I still get offered the chance to beta test games.
I think SCEA directs specific surveys to GAP members and there is a specific forum for GAP members.