To explain this to the uninitiated: (First follow another explanation of the original combo)

In Magic spells are not in effect immediately, instead they go on the stack, before they are resolved. Spells from a stack are resolved in LiFo order, so last in, first out. Instants, like Gut Shot, can be put on the stack at any time by any player (simplified).

So Enemy gets his 21 mana with channel, you let that resolve, he has 1 life. He pumps it all into the fireball, now you act. While fireball is on the stack you cast Gut Shot, paying with 2 life of your 20. It goes on the stack. Your enemy hopefully has no response, without any mana left in his pool, and he looses his last life and the game.

When a player looses the game all his spells are removed from the stack, so you really win and there is no draw.

You stopped his combo with 4 cards worth unreal amounts of money with a single card worth about 50ct.