Whereas a traditional gimlet includes gin, lime juice, and simple syrup, a French gimlet swaps the simple syrup for St-Germain, the sweet and syrupy elderflower liquor. It is a wonderful drink for a hot day.
Some people also add a splash of Lillet Blanc. I like the simplicity of just three ingredients, but I also like Lillet, so it is up to you. Try one of each!