Hard to say with Gambit's appearance in Grand Design, years of stories are crammed into a single comic. So maybe just a few panels. But yeah, he'll be there at the very least.
As for Hickman, Gambit not in the big Mark Brooks promo art (which has literally 30-40 characters), is pretty much a dead giveaway that he'll play no role in the event, maybe a satellite book by a different writer is what JDW is hinting at. I'm sure Hickman will write the series well, but from what I hear, it's focused on Professor X, Magneto, and there's 3 Wolverines and I'm sure Cyclops will play a big part as well...so pretty much the same thing for the last 20 years. But this re-launch is for realz! Like I said, Hickman will probably do a fine job and good luck to him, but I'll wait and see before I judge it properly...and not to listen to the hype and the sycophants that are prematurely hopping on the bandwagon.