Puree the watermelon, sugar, lime juice and Sambuca in a blender until smooth, working in batches if necessary. Pour through a fine-mesh sieve into a large liquid measuring cup.
Pour 1 cup of the watermelon mixture into a small saucepan and bring to a boil. Meanwhile, pour another 1 cup of the watermelon mixture into a large bowl and sprinkle the gelatin on top; let stand 1 minute. Pour the hot watermelon mixture into the bowl and stir until the gelatin dissolves. Stir in the remaining watermelon mixture from the measuring cup.
Spray an 8-inch-square cake pan with cooking spray, wiping out the excess. Pour in the watermelon mixture. Skim the foam from the surface and refrigerate the gelatin until slightly thickened, about 20 minutes. Arrange the mint leaves on top in rows, then push them just below the surface. Refrigerate until fully set, at least 4 hours.
Unmold the gelatin: Invert onto a cutting board, then re-invert onto another cutting board and cut into squares.