Make the simple syrup: Heat the sugar with 1/4 cup water in a small saucepan over medium-high heat, stirring, until the sugar dissolves; let cool.
Combine the gin, lemon juice, cherry syrup and 1/4 ounce simple syrup in a shaker with ice; shake well. Strain into a coupe glass. Top with the sparkling wine. Garnish with a lemon slice and maraschino cherry.