Possibly nothing, the Gatwick operation offers nothing whatsoever to Delta, so why would they bankroll a multi million $ replacement when they serve the House of Mouse via ATL. Like BA, LGW has been the poor cousin of LHR. Was left with only leisure routes, then second hand B742s, second hand Icelandic B742s, then eventually B744s only because they got a cancelled Alitalia order.
So on past form, LGW will get the ex LHR A330s, or nothing, especially now there's no synergy with Delta.