I think Brook is bigger and probably better than people are giving him credit for, but overall I think GGG is a bad matchup for Brook. Brook's best competition so far has been Porter, who is a fairly big/strong guy who stalks and moves forward like Golovkin, but that is pretty much all Porter and GGG has in common.
When Porter got in close, Brook would make Porter smother himself and then tie him up. He had the strength to turn Porter in the clinch, to put himself in a better position. He'd also bait Porter into swinging wildly, making him easy to counter then hold. Sometimes he'd intercept Porter's punches with a jab or lead hook that would throw Porter's shots off target (then smother/clinch).
GGG controls distance too well for Kell to manipulate like that, from what I've seen. I have never seen GGG completely lose control of distance, he always seems like he knows where he needs to be, unlike Porter who charges like a bull. Plus he is too strong for Kell to tie up, and he knows all the little tricks to control positions inside so he's very hard to clinch. It's also not like Kell is going to manhandle GGG even if he does manage to tie him up, GGG will probably be able to maneuver out of the clinch before the ref even separates. We also know that you're not going to interrupt or misdirect a GGG bomb very easily by beating him to the punch, which Geale learned the hard way.
I just don't see what Brook does that will get him the win. I can definitely see him giving GGG some trouble with his quickness and timing. I can see Brook landed some good strong jabs and lead left hooks at some point. I can see him landing some good counters with his left hand, and I think he might even bust GGG up a bit before he gets hurt and likely stopped. He could move backwards the whole fight and might be able to survive, but he won't give himself a chance to win if he does that.
