Bring cranberries, sugar, and water to a simmer in a 3- to 3½-quart saucepan, then simmer, uncovered, stirring occasionally, until berries just begin to pop, about 2 minutes.
Drain cranberries in a fine-mesh sieve set over a 1-quart glass measuring cup, then reserve 2 cups cranberries and force remaining berries through sieve into syrup.
Discard solids remaining in sieve, then add reserved cranberries to syrup.
Cool to room temperature. Refrigerate until cold, about 2 hours.
Select your serving preference:For a pitcher: Transfer syrup to a pitcher. Add remaining ingredients to syrup, stirring gently to combine. Serve drinks over ice in 8- to 10-ounce glasses and garnished with a lime slice and some whole cranberries (from syrup).For individual servings: Place 1 ounce of gin in a 8- to 10-ounce glass filled with ice. Add 2 tablespoons cranberry syrup (or to taste), 1 teaspoon fresh lime juice and top off with tonic, stirring gently to combine. Garnish with a lime slice and some whole cranberries (from syrup).
Notes
* Cranberry syrup can be made a day or two ahead and keeps for a few weeks in the refrigerator. * I recommend using Q Tonic Water or Fever-Tree Indian Tonic Water for the best quality drink.
Recipe by Riegl Palate at https://www.rieglpalate.com/cranberry-gin-and-tonics/