You're right, Gastelum has looked great since the Magny fight (which was actually closer to 1.5 years ago than 2 years), but he's also fought 2 fighters who have been looking awful lately in Vitor and Hendricks and another fighter well past it who was coming off a 2+ year lay off in Kennedy. I do think that Gastelum has improved, but I'd still like to see how he handles a top flight 185'er before I can say for sure he's a contender at this weight class.
Also, if anything I think what happened in the Magny fight gives me more confidence in Weidman's ability to get Kelvin down. It's not like Gas has developed into a true MW (like Whittaker has for example), but he just didn't have the discipline to make 170 and I believe he's admitted that himself.
Kelvin was pretty reckless working to get back to his feet in the Magny fight and I think Chris might be able to make him pay where Magny didn't. If not, I'll be looking to live bet Gastelum.