Straight out of a Raymond Chandler novel, this is traditionally done with simply gin and Rose's Lime Juice. I like Ted Haigh's version where he does half Rose's and half fresh lime juice as it's much less sweet.
2 oz. gin
1/2 oz. Rose's Lime
1/2 oz. fresh lime
Combine ingredients into a shaker. Add ice, shake vigorously and strain into a cocktail glass.
"A real gimlet is half gin and half Rose's Lime Juice and nothing else. It beats martinis hollow." - Raymond Chandler, The Long Goodbye