This all depends on how Kelvin does with his weight cut. IF, and we all know this is a huge IF, he cuts weight properly and comes in ready to go, he will grapplefuck Brown into a living death, and may even get the submission. This kid has the potential to be the champ at WW, assuming his body is capable of consistently making 170. His wrestling, standup, toughness, and even his conditioning (when he doesn't come in half dead from dehydration) are all first rate and he might have the most underrated submission game in the division right now. He's an awesome fighter.
I'm even a believer in Gastelum at MW, and I definitely believe he will be a force at 170 if he can actually make the weight, although I am really not sure he's capable of that. Matt Brown is a first rate striker who still struggles mightily against excellent wrestlers. He's tough as nails and will show up to throw down and if Kelvin is in the kind of condition we saw him in against Woodley, he'll finish Kelvin for sure.
This is a very interesting fight but I see it as being Kelvin's to lose (which he very well may).