GGG might give the impression of being basically a middleweight Marciano, but the huge flaw in GGG is that he has long arms and is therefore not very good on the inside. Marciano had very short arms. He would get inside on you and then you were hopeless. Roland LaStarza and Ezzard Charles had good footwork and were able to keep him at bay enough to go the distance and make it a great fight, but the other guys got brutalized.
Marciano never really jumped on hurt opponents,you almost had to cooperate with him by just standing there for him to tee off on you..golovkin is a finisher and if he gets you hurt he goes in for the kill...not really Hw's here...one was 160 ,the other 185,both have Heavy power...it would all come down to chin. Golovkin the much better boxer combined with power makes him not one dimensional here..Marciano would have to break him down,but I would lean toward Golovkin and my guess he would fight at 170 here.