Although there is no direct answer I will try to explain it using available info and logic. With G+B fail you loose left elevator and slats. With only half elevator to create the required aerodynamic force it has to move double the amount it would move with both sides working. Besides being on one side of longitudinal axis it causes rotary motion like an aileron which needs to be resisted causing torsion on the fuselage. So elevator handling needs to be gentle and only to the required amount. In normal/alternate law fore and aft side stick movement is linear load factor demand which flight control computer gives by moving the elevator. So there is a possibility of elevator hitting the stops causing damage so the aircraft automatically restricts the maximum movement. At high speed because of V square it is not a problem but at lower speed a certain G demand may move the elevators to extremes. This also aggravates torsion. This can be prevented by inducing direct law and moving the stick/elevator to required amount and also being gentle. Harsh stick inputs cause false stall warnings which pilot doesn't know as false. Therefore first gear down at two hundred kts. Now why not with slats? Because to get slats you would be in normal law may be with single hydraulic failure and your speed would be closer towards S which is correct speed till now. So if double failure occurs now you are already there and can continue to flap2 and gear down(gravity extension) and flap3.