Bentley was penalised for bringing down a Gills striker and with no substitute keeper on the bench, midfielder Jamie Dennis placed on the green jersey.
He proceded to pull off a fantastic save diving to his left, giving the Under-18's the win as they continued their unbeaten start to the season.
Head of Youth Ricky Duncan told the official site: "Jamie Dennis has always wanted to go in goal and he stepped up. He keeps saying in training 'let me go in goal' and to be fair he pulled off a great save low down to his left-hand side. It was a good penalty, so he did well to save it. He also saved a free kick at the end, so he'll be pleased as he managed to get the three points."