Fill a pitcher adding the gin, grapefruit juice, lime juice, bitters and cilantro or (mint). Stir with a wooden spoon until mixed well and gently press the sprigs against the bottom of the pitcher a few times to muddle the herbs. Pour into 6-8 old-fashioned glasses (only fill 2/3 of the way) filled with several ice cubes. Top with the tonic water and give the drinks a quick stir. Garnish each glass with a cilantro or mint sprig and serve. Recipe from Food & Style.