Im pretty sure they will revoke the redeemed games and possibly ban the MS/xbox accounts that redeemed them.....
Since it works on xbox one as well they might ban the consoles the xbox account was linked to.
There is virtually no chance they ban anyone. There's no way to tell which redemptions were from people who were duped buying third party from those who knew they were illegitimate since its not illegal to sell them if obtained legitimately. Considering MS are behind Sony right now and Windows Store is a relatively new PC platform for AAA games, its even less likely as they risk losing customers on two platforms by mass banning.
The worst case scenario for anyone who redeemed a glitched code is that their game is revoked and they lose whatever they paid for the code unless that seller is nice enough to provide a refund. Glitches have happened before on MS platforms and revocation is possible, but there's no need to worry about being banned. That's Chicken Little nonsense.
Also to add to this, Nvidia is now requiring proof of purchase to redeem any coupons in this promotion, so if you do plan on buying one in the future, make sure you ask your seller for that proof.
All un-redeemed NVIDIA coupons for Gears of War 4 obtained thought the glitch has been disabled
The activated coupons e.g. the actual game will remain in your account, but future coupons from the glitch won't work. (You can try)