Combine sugar, grapefruit juice and lemon juice in large pot. Bring to a rolling boil, stirring constantly. Stir in pectin. Return to a full boil and boil 1 minute. Remove from heat. Quickly skim off foam with a metal spoon. Ladle hot jelly into sterilized canning jars. Leave ¼ in headspace. Wipe rims and add lid and ring. Process in boiling water for 5 minutes. Great on toast, biscuits, cream cheese and crackers.