For the Oleo Saccharum: In a mixing glass, add sugar. Peel a grapefruit and two lemons in long, thin ribbons and add them to the sugar. Muddle the citrus peels, pushing the oils from the peel into the sugar. Over time, the oils will dissolve the sugar making a thick syrup. Cover and let rest overnight so the syrup can be fully infused with the citrus peels.
For the punch: Place the oleo saccharum into a punch bowl along with the lemon and pineapple juice and stir. Add gin and a bottle of champagne.
Place a large piece of ice into the punch (fill a plastic container with water, freeze, then unmold). Serve.