In a small bowl, whisk together 2/3 cup grapefruit juice, Campari, and gelatin. Let sit for 5 minutes.
Meanwhile, in a medium saucepan, combine remaining 2/³ cup grapefruit juice and 1½ cups sugar. Bring to a boil over medium-high heat. Cook, stirring constantly, until sugar dissolves, about 5 minutes. Add jelly and vanilla, and whisk to combine. Return to boil, and cook until thick, about 2 minutes.
Add gelatin mixture to jelly mixture, whisking to combine. Strain mixture into an 8x11-inch baking pan. Chill for 4 hours or overnight. Cut into squares.
In a small bowl, place remaining 1 cup sugar. Toss jellies in sugar to coat. Garnish with edible gold leaf, if desired. Store covered in refrigerator for up to 1 week.