I'm talking about the smooth rock you find that opens up the path leading to the Flower Paradise if you've managed to get Oak's Letter. The one where you have to write down the person you're most grateful to. I've had the idea of Ash finding that stone and needing to get past it.
He nearly chooses Pikachu, but is encouraged to not be so hasty in his decision, since it's one of the most important ones he'll ever make. After some thinking, he realizes that it's Gary he's most thankful for, since if it wasn't for Gary pushing him and taunting him into wanting to become a trainer just to beat him, he never would have met Pikachu or any of his other friends.