If 12Bxg5 black has 12..Qc5 forking the 2 bishops. The only way to avoid losing a piece I think is 13h3 but then 13..Qxg5 14hxg4 Bxg5 gets the pawn back and black has a nice looking attack building up, where is white's Q to go for example? Black can use the now open g file etc.