Make the simple syrup: Bring the sugar and 1/2 cup water to a boil in a saucepan over medium heat, stirring until the sugar dissolves; let cool.
Combine the orange juice, lemon juice and 1/2 cup simple syrup in a punch bowl. Add the gin and elderflower liqueur and stir. Fill with ice, then add the champagne. Garnish with lemons, strawberries and pepper.