And as far as i know, GMG is an official reseller for 2K games, so they get their keys directly from them, which means there shouldn't be a (noticeable) delay.
I got XCom2 from them and i know steam purchasers were able to pre-load slightly before i could, but i was still able to play on day1 (or rather, install on day1 and then find out my GPU wasn't up to the task, but obviously that one was on me). Haven't bought Civ6 yet, but i'll get it from them sooner rather than later.
That being said, buying from a 3rd party site rather than steam directly often means you're likely to get the game slightly later. The question is, do you accept a delay of a few hours or a couple days (at worse) for 20-25% savings. I'll take the savings any day for any game, but some people don't want to wait and have more money to spend. As long as you avoid shady stores selling dubious keys, you'll be fine.