Gibbs-White fooled United goalkeeper Andre Onana to score Forest's second in their 3-2 win on Saturday.
Onana's blunder surprised the midfielder, who said afterwards: "I didn't (think it was going in) at all, to be fair.
"I don't want to discredit Onana, he's an incredible goalkeeper, but when I had the shot, I thought he was going to pick it up.
"But hey, you count them don't you, I'm happy to contribute to the team."