The rules of Gin Rummy
Two players, one deck of 52, ten cards each. One card is turned up: the player who did not deal may take it or pass, then the dealer; if both pass, the first player draws from the stock.
On your turn, draw from the stock or take the top of the discard pile, then put one card down. You may not put back the card you just took from the pile.
Meld your cards into runs (three or more of a suit in order, the ace low only: A-2-3, never Q-K-A) and sets (three or four of a kind). Cards in no meld are deadwood: an ace counts 1, a picture card 10, any other card its number.
When your deadwood is 10 or less, knock instead of putting a card down. Both hands are laid out; the other player lays off what they can on your melds. You score the difference between the two deadwoods. If theirs is no more than yours, they undercut you and score the difference and the undercut bonus. With no deadwood at all you go gin: no lay-offs, and the gin bonus on top of their deadwood.
When only two cards are left in the stock and nobody knocks, the hand is void. The winner of a hand deals the next. The first to the target wins the game, with a bonus for each hand won and the game bonus; a shutout doubles it.