The gin sling recipe dates back to at least the late nineteenth century. This dusted-off version comes from dale degroffs seminal tome, the craft of the cocktail. Degroff has a sweet tooth, and we often find that we need to reduce the sweetness in his recipes a bit to appeal to post-millennium tastebuds. We figured wed have to do that here, with such a large serving of simple syrup. We were pleasantly surprised to find it was quite tasty as written; the sparkling water helps to keep the sweetness in check.