Combine first 3 ingredients; stir well. Bring to a boil; cook, stirring constantly, 1 minute or until sugar dissolves.
Combine sugar mixture, grapefruit juice, and tequila in a large bowl; stir well. Pour into a 13- x 9- x 2-inch baking dish. Cover; freeze 8 hours or until firm. Remove from freezer; scrape with tines of a fork until fluffy.