Nothing has yet been confirmed. It is likely that there will be a pre-flight dining area for First passengers, if they don't provide a separate Galleries First lounge (which I can't see them doing with only a few flight departures).

All the other oneworld lounges will still be open, and according to oneworld access rules, the new Galleries lounge should be accessible to all premium oneworld passengers.

If I were travelling in First or a Gold member, I'd be tempted to visit one of the other oneworld first class lounges, rather than 'slumming it' with those pesky business class passengers.