Cerdan was strong but was not a big middleweight. Golovkin would be coming in with a size advantage. The quicker Cerdan may be able to box and fight it out some when needed to a decision win, or the heavier punching of GGG may take its toll late and bring him back. Close fight but I feel it's really too hard to call confidently.
GGG hands down. GGG would have fought at light heavy had he fought in the same day weigh in era. Cerdan had a record padded by a lot of unknown Europeans. Don't get me wrong, he was good fighter his era but not on GGG's level.
I don't underestimate Cerdan but I reckon GGG wins this. Not an easy fight for him at all but I see him getting a points win.