All of which brings us to the third kind of “gams,” slang for a woman’s legs, especially if regarded as attractive. “Gam” in this sense probably reminds most people of the “noir” crime novels and films of the 1930s and 40s and the hardboiled patois of Raymond Chandler, Dashiell Hammett and James M. Cain (”The gams! The gams! Your face ain’t news!”, Mildred Pierce, 1941). But “gam” in this sense is actually considerably older than Sam Spade, dating back to at least the late 18th century. And “gam,” which began as underworld slang, originally referred to the leg of either sex, and not necessarily an attractive one
http://www.word-detective.com/2009/04/24/gams/