Place grapefruit segments in an even layer on a double thickness of paper towels. Using more paper towels, gently press tops of segments to soak up excess juice. Remove top layer of paper towels and let grapefruit air dry, about 1 hour. Line six 8-oz. ramekins with 2 layers plastic wrap, letting 2” hang over the edges. Arrange grapefruit in layers between ramekins, alternating colors; set aside.
Place gelatin in a bowl and cover with 2 cups cold water; let sit until soft, 3–5 minutes. Meanwhile, combine grapefruit juices, sugar, Cointreau, cinnamon, and salt in a 2-qt. saucepan; bring to a boil. Cook until sugar is dissolved, 2–3 minutes; remove from heat. Squeeze water completely from gelatin; whisk into sugar mixture until smooth. Divide evenly between ramekins; chill until set, at least 6 hours. To serve, unmold terrines; peel off plastic wrap.