The 17-year-old forward had only been on the pitch for three minutes after he replaced Gavin Massey.
But Gelhardt made an immediate impact for the visitors with a superb turn and strike after 75 minutes.
Cook said: "He's got that unique talent to find space in a telephone box.
"He's such a natural, good footballer. To get a goal like that in front of his own fans was a special moment.
"A lot of young footballers coming into the game are not very nice people. We're just lucky to have a boy with his humility playing for us – the game is easy for him.
"He'll be starting soon if he carries on like that. He's a young boy that needs minding though.
"It's a very special goal from a very special player."
Wigan continue to struggle on their travels – they have won just two of their last 29 games away from the DW Stadium – but Cook feels his side are making progress.
He said: "For us to take the lead, and go behind in the fashion that we did, it was pleasing for us to come back.
"I enjoyed watching us play. I thought we were full value for at least a point.
"Belief comes when your confidence is high. It's only a point but we must now carry that on to the next game at Charlton.
"If we keep doing what we do, we'll get there.
"Hull have got a good home record and are a good side, but we managed the game well.
"The second goal epitomised what we are trying to do."
Wigan took the lead when Chey Dunkley seized upon George Long's error after eight minutes.
Jarrod Bowen equalised two minutes later, while Kamil Grosicki put the hosts in front with a well-flighted free-kick from the left corner of Wigan's penalty box.
Grosicki's 20th-minute goal gave Hull momentum which they carried into the second half, but Gelhardt had the final word.
Hull have now not won in the Sky Bet Championship since a 2-1 defeat of Reading on August 10, but manager Grant McCann is convinced his team's fortunes will soon change.
He said: "It (a return to winning ways) will come. We're not worried at all.
"I'm frustrated with the result as I thought we created enough opportunities to win the game. To score two goals at home, we should be winning the game.
"The first goal we were annoyed about, but the second one was really, really frustrating for us. We should have defended it much better.
"It feels a bit like a defeat, to be honest, as I thought we had opportunities to kill the game."
McCann added: "Overall, in large periods, I was pleased with how we played – we attacked well and broke well and played on the front foot.
"But football is hard work. You have to work, run and fight in my teams and I think, for split seconds, we are switching off.
"We've definitely got the personnel to play how I want us to play, but it's my job to make sure they do it for 90 minutes."