The rules of the game depends on the variation of Gin Rummy that is being played. Listed below are the most common variations, and the only ones that I usually played online.  By far the most common scoring system for gin rummy is the 20/25 system of scoring, 10/20 system is more traditional and is second in popularity.

10/20 system

If the knocking player has gone gin, he scores 20 points, plus the deadwood count of the defending player.

If the knocking player has not gone gin, and the defending player has an equal or lower deadwood count, he has undercut the knocker, and scores 10 points plus the margin by which his deadwood count was lower.

If the knocking player has not gone gin and is not undercut, he scores according to the margin by which his deadwood count was lower than that of the defending player.

20/25 system

This system is the same as that above, except for that the gin bonus is worth 25 points instead of 20, and the undercutting bonus is 20 points instead of 10.

Single match

When a single match is to be played, the players will continue to play rounds until one player has 100 points or more. This player wins the match.

Multi-match

In multi-match games, it is typical to keep track of both match and game scores. Match scores are reset to zero with the start of each match, but the game scores accumulate from match to match, and are ultimately used to determine the winner. A match ends when one player scores 100 match points.

At the end of the match, players' match scores are credited toward their game scores, as well as:

• 25 game points for each round won,
• 100 game points to the winner of the match, and
• 100 additional game points to the match winner if the loser won no rounds.