I think Brook has to use what I believe to be advantages in speed and timing. Beat Golovkin to the jab, and then move immediately. Use unusual angles. Smother when in close like he did Porter. Move. Smother, get his shots off on the inside, Golovkin is poor on the inside, (even his fans aren't blind to this) move. It will require a superhuman effort, it's unlikely but it can be done. Golovkin also is somebody who's face marks up so Brook needs to target the areas he is having success in. It's a 15% chance no more but Brook has a chance. I genuinely feel Golovkin's lack of elite opponents (Martin Murray is still tellingly his actual best win, not great) might be an issue if it goes long. Move. Move.
If by "put away" you mean knocked out or stopped, then to answer your question, probably Vyasheslav Senchenko (at 147) or Carson Jones in the rematch (at 152 CW). Brook won't disgrace himself, sure; but Golovkin, as well as being a natural Middleweight, is several levels above anyone Brook has ever faced.
I'm going to leave this here. I feel Brook will acquitt himself very well vs a man 2 weight divisions above him and - by casuals only admittedly - rated as the number 1 p4p boxer. Personally I think beating Martin Murray and Matthew Macklin shouldn't rank you anywhere near the league of Ward, Kovalev, Pacquiao, Klitschko, Fury, Gonzales etc I guess some fans have an INCREDIBLY high valuation of Macklin and Murray. I don't. So I think Kell will do quite well, but lose ultimately. Of which there will be no shame.