Stir together rosé wine & sugar in a small saucepan over low heat. Stir occasionally until the sugar dissolves. Remove from heat & let the syrup come to room temperatature before using.
Combine gin, ¼ cup of rosé syrup & lemon juice in a cocktail shaker. Fill with ice & shake vigourously. Pour into a chilled champagne glass (I prefer the coupe style). Top with the chilled sparkling rosé to taste. Garnish with the lemon slice and serve immeaditaely.