Today I computed the stats for Gin Rummy, same as for Euchre: I only included in the stats those players who had completed at least 100 games. I have used the numbers that produce the "Absolute win/loss ratio" shown under your "stats" link. The histogram is shown below. There are two players who attained a win ratio of > 70%! Most players come in around 40-50%.
I think the better your memory, the better you will do at Gin Rummy. I keep playing in the hopes that it will improve my memory! :)