GGG probably finishes Brook but something isn't sitting right with me about this one. Golovkin on a 22 fight ko streak or something like that, fighting a guy who's jumping up two weight classes, you think GGG probably gets him out of there at some point, seems pretty straightforward.
That being said I am having some second thoughts on it, firstly Brook is a giant 147, cuts a lot of weight to make the limit, he could easily fight at 154 and hold his own so it's not like he's some tiny guy fighting a giant, they're closer in size than we'd first think.
Secondly, Brook is a very skilled fighter and is one of the most skilled guys that Golovkin will have fought in the pros, even with the weight discrepancy, which as mentioned, isn't as bad as it seems at first glance. He could very well have some legit success boxing and tying GGG up, keeping him out of his rhythm here, albeit in between bouts of getting his ass kicked.
All in all I think Brook could make this pretty tough for GGG but he's definitely gonna take some serious shots in the meantime. I don't think he can beat GGG here, he's very skilled but Golovkin is just too powerful and too well rounded with his offense, I don't think Brook will be able to pull off a big upset here, just a matter of if he survives or not, imo.
Taking everything into account, I'm assuming GGG gets Kell out of there, even if it takes him longer than usual. That being said my gut is keeping me from laying the juice atm based on Brook's skill and the fact that he's not gonna be all that much smaller than Golovkin. I think Brook will have some success boxing as mentioned, he's gonna take a beating but I think he's got the ability to get some good work done, maybe win a couple rds in the process, most importantly survive for longer than most guys do against GGG. Thinking about the over as of now, depending on rd and price, obviously. maybe GGG 7-12 although I doubt there's gonna be a whole lot of value on that one either.
tl;dr - Not betting anything yet. I think Golovkin SHOULD get the finish here, but thinking that Brook can survive some early onslaughts and make it deeper into the fight than most.