As above, the GGL team can request an additional A class seat if needed. So if there are 5 unsold F seats in the cabin but the buckets are showing F5A1 and you need 2 seats, they may well be able to get one of the F into A so you can book.
I doubt they'd do it with the last available seat unless it was very close to departure, but may be route dependent. LAX - no chance. ATL - maybe.
However if it's F1A1 and you need 4 seats, then I'm afraid they can't install more seats on the plane for you.