Combine juice, sugar, and Grand Marnier. Let stand 30 minutes, stirring often. Pour mixture into an 8-inch square baking pan. Freeze 2 hours or until firm, stirring after 1 hour. Remove mixture from freezer; scrape entire mixture with a fork until fluffy.
Substitute 2 tablespoons of tangerine juice, lemon juice, lime juice, or orange juice for the Grand Marnier if you prefer.