Put the gin, grapefruit juice, lemon juice, Simple Syrup, Chartreuse and orange bitters in a cocktail shaker, add large ice and shake vigorously. Double-strain into a Nick and Nora glass (or other cocktail glass) and garnish with a grapefruit peel twist.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.