Here is the full list of all 12 Gin Rummy achievements worth 200 gamerscore. It takes around 20-25 hours to unlock all of the achievements on Xbox 360.
Win 1 round by going Gin
Win 1 round by Undercut
Win 1 round by Knock
Finish a 4-player game of Gin Rummy in any mode on Xbox LIVE
Win 1 round with only sets
Win 1 round with cards including deadwood from only one suit
Win 20 rounds by going Gin in any game mode
Win 1 round without drawing from the discard pile
Win 10 matches or games in Single Player.
Win 1 Player Match hand against one player by 50 or more points
Win 5 games/matches in each single player mode, and 25 player matches
Earn over 50,000 Total Points