Here's what's gonna happen:
1. You're gonna get a response from GOG Support and they'll say you're SOL. However, this will take 2+ weeks.
2. Then you're going to go back to Instant Gaming and try to get a refund. Then they'll also say you're SOL since their return policy is only 14 days and they know how swamped GOG Support is.
3. Most people will give up here and write it as a loss because they don't know how to do CC chargebacks or don't want these chargebacks getting them banned from these grey market sites because their "good" prices are too good to give up.
You're better off processing a refund from Instant Gaming right now and getting an actual key from GOG instead of buying from these grey market sites. I keep reminding people that you don't know where these keys are sourced and if they're bought with stolen CCs, that key you bought will be revoked from your library in the future, cost the developer time and money during the bank investigative process, and you're left with the loss with no game to show for. Not to mention other stuff like inadvertently supporting organized crime, shady business models, and/or exploiting regional pricing unfairly.
Think - why would Instant Gaming tell you to check with GOG Support on the key status when it was the former who sold the key to you? Instant Gaming should be the ones ensuring that the keys haven't been redeemed yet before selling them to customers if they were a legitimate business.