Hagler is one of my top 3 all time favorite boxer, I still voted draw and I explain why; Hagler was a legend in itself, an adamantium chinned god of perfect movement on the ring and he fought against the hell of an opposition. It's always hard to pit a boxer from today against such a legend of the past, especially because of the halo that comes with it.
On paper, I think that GGG has wonderful tools that could have fared very well against Marvelous. Enough to beat him? That's where it gets hard to tell; GGG hasn't fought enough top of the league competitors and his best years are slowly crawling behind him. We haven't seen enough to know for sure, but there's no doubt that the kid's special.

For that reason, the lack of data vs the legend, I'll be careful and speculate that a draw could be possible.