After seeing pics of Hagler stood beside Golovkin it's clear to see he is a much smaller man even though he did have massive fists for his size. With 24-hour weigh-ins I believe he could have made 147lb. I'm not saying he would stay at 147lb but to face the likes of GGG at 160lb he would be facing much bigger men in today's climate.
It's hard to say because his whole approach to making weight, from his first amateur fight to the Leonard fight, was based on the idea of weighing in the day of the fight. If he started in 2010, his whole approach to weight management would be entirely different. The idea of finding your natural weight class and staying there wouldn't ever be in his mind. It would be replaced by the prospect of cutting as low as possible and fighting in multiple divisions.
Hagler never looked big standing next to his opponents. He usually looked smaller. Hagler was a small/medium-framed middleweight with good solid muscle.
GGG is clearly a much heavier man IMO. It's not a stretch to say he could have made 147lb in today's climate. I have no doubts with today's vitamins and minerals he would have been a champion in more than one division.
I weighed in at 157lb quite often in fights so he could easily make 147lb with 24 hour weigh-in's. Or maybe he would have stayed at 160lb and beat naturally bigger fighters regardless.