To preserve the Green Hydraulic System integrity (FCOM 3.02.29).
Correct.
Both slats and flaps are now being supplied by the GREEN System, the extra (Enormous) hydraulic load from operating the gear would not be prudent with BLUE and YELLOW out.
Don't think this is the reason - this is more to do with isolating the landing gear hydraulics in case there is an unknown leak downstream of the Safety Valve and Cut Off Valve. Using Gravity extension isolates the gear hydraulics.