In a small saucepan, bring sugar and 1½ cups water to a boil over medium-high heat. Reduce heat and simmer until sugar dissolves. Let cool to room temperature.

In a large pitcher, whisk together cooled simple syrup and lemon juice. Fill each serving glass with ice. Divide lemon juice mixture between glasses. Add gin, if desired, and stir. Top each evenly with sparkling water. Top each with grenadine. Garnish with cherries and lemon, if desired.