You can make this cocktail with homemade grape puree, or store-bought 100% concord grape juice (look for a bottle that lists nothing but concord grapes in the ingredients, not even water). I can't say what the difference is between the two -- they're both made from pure concord grapes -- just that there is a difference, and it's a big one. The fresh puree has a lighter flavor and retains more of the fresh concord grape tartness, while the store-bought juice has more of a "grape popsicle" flavor, as I like to call it; sweeter, and somehow more "grapey." I definitely prefer the homemade puree, but both are delicious, and the store-bought juice will work in a pinch if concord grapes are too pricey or just out of season. That said, if you're using store-bought juice in place of the puree, I recommend using a little less simple syrup to make up for the sweetness.
For the simple syrup, combine equal parts sugar and water (1 cup each) in a small saucepan or microwave safe container, and heat until sugar has dissolved. Once cool, syrup can be stored in a jar in the fridge and used for future cocktails. Feel free to adjust the amount in this recipe according to your tastes (I like ½ an ounce if I'm using homemade concord grape puree, or just ¼ ounce if I'm using store-bought grape juice).
You can also feel free to swap the rosemary for any other herb you like or happen to have. I tried using fresh sage, and also lavender, and both were delicious.
Cocktail adapted from and inspired by this
grape gimlet from Gramercy Tavern, via Serious Eats.