In a pitcher, muddle sugar cubes with club soda until the sugar is broken up. Add the gin, vermouth, grapefruit juice, and lemon juice and fill the pitcher 3/4 full with ice. Stir 50 times and strain into a punch bowl. Add one large block of ice. Top with Champagne (or another dry sparkling wine). Garnish with the grapefruit wheels and serve with a ladle and punch glasses. Recipe makes 4 servings.