Add gin, lemon juice, and simple syrup to a cocktail shaker then fill 3/4 of the way up with ice.
Shake until well chilled then strain into two glasses. Top with champagne then serve.
Notes
To make simple syrup: combine 1/2 cup each granulated sugar and water in a glass measuring cup or jar then microwave for 1-1/2 minutes. Stir until the sugar has dissolved. OR combine sugar and water in a small saucepan over medium-heath then stir until the sugar has dissolved. Chill completely before using — can be made up to 2 weeks ahead of time.