I really can't say before I've seen Golovkin against better opposition. Hagler would be one he in all likelihood wouldn't KO, so it would be an entirely new kind of opponent for him.
We know that Hagler could hang with and ultimately break down very tough and hard hitting boxer punchers like Mugabi, but Golvkin hasn't been up against someone against whom his power isn't enough, but where he'll rather have to rely on skill, smarts and sheer raw will to win. He certainly would have to against Hagler. So I just don't think we have the facts to something very useful about this fantasy match-up.
I think a fighter who can't be hurt beats a puncher. . I don't think you can hurt hagler. I think he'd out box ggg
Hard to say before we've seen Golovkin really have to box. He clearly have good technical skills, but they haven't been seriously tested. Hagler didn't have boxing skills to match the absolutely best boxers imo. He was more skilled than the brawlers and punchers and stronger and more powerful than the boxers of his time. That was enough until he ran into Leonard. As for Golovkin we just don't know how he would do against a skilled boxer-puncher with Hagler's work-rate, chin and brutal will to win at any cost.
Hagler should come through a very hard fight and win if hearns and the beast couldn't KO Marvin I doubt GGG could. That said we have yet to see GGG going all out to win so who knows?.