John Trupin at Lookout Landing came out with a great post a couple of weeks ago about Marco's arm-slot change.
https://www.lookoutlanding.com/2018/2/19/17025530/marco-gonzales-lowered...
TL;DR - Marco's arm slot change has unlocked a few extra MPH in all of his pitches at the expense of his impact change-up.
Given his current mechanics, we will never see his deadly change of yore again. And that's okay! I think Marco's repitoire is more sustainable as a James Paxton lite lite lite than as a...whatever he was before: short-arming, over-the-top, control lefty struggling at the edge of the acceptable velocity window.
I'm looking forward to watching Marco pitch, now. I was really pretty raw about the O'Niell trade. This mechanical change has me intrigued.