Combine 3 1/2 tablespoons cold water and gelatin in a small microwave-safe bowl; let stand until gelatin starts to dissolve, about 5 minutes. Warm the water in the microwave to dissolve the gelatin completely, 5 to 10 seconds.
Bring 1/2 cup water to a boil in a small pot. Stir in rock sugar until dissolved. Stir in lychees and 1 1/4 cup reserved syrup.
Combine gelatin mixture, lychee mixture, and 1 cup plus 1 1/2 tablespoons milk in a bowl. Pour into a container and chill until set, about 4 hours.