Lurking around the 'other' forum, I've seen a couple threads on what seem to be serious engine / tranny issues on some 2018 GTs. And I said 'seem', they're still at the dealer.
One guy's engine just stopped running and is about to be opened up. Someone mentioned another car had a busted valve.
One member posted a video of his tranny grinding when shifting above 5k RPMs:
Then there's a whining issue popping for others.
First year production woes most likely which can apply to any manufacturer, model. Still, if I was in the market, I'd hold off for a while till the dust settles.