This gin drink is a jewel of a cocktail. You can play with the measurements somewhat. Try it with equal parts gin, vermouth and Chartreuse if you like. I like a bit more gin myself.
Chill a martini glass in the freezer. Add ice to a cocktail pitcher. Pour in gin, sweet vermouth, and Chartreuse. Shake in bitters and stir. Strain into martini glass and garnish with a twist of lemon.