I think the big difference between Hagler and GGG would be that Hagler was a more versatile infighter than GGG is. That's not to say that GGG can't fight inside, but Hagler was a master there. Hagler was also a switch hitter, which means he could trouble GGG from any angle.
GGG is more versatile than given credit for though, he has a pole for a jab, good slipping ability when on the offensive, and has been seen to fight on the backfoot against Lemieux. He's also got good ring cutting skills, which would limit Hagler's ability to fight on the back foot to some degree.
If Hagler and GGG were to go at it, I favor Hagler by late stoppage or UD, but he'd have to go through absolute hell to get there: nobody is getting into a firefight with GGG and making it out without at least significant damage, particularly not the early GGG we saw fight around the Proksa era.