Slice lemons in half lengthwise. Juice lemons until you get 2 tablespoons juice. Hollow out both the juiced rinds and the remaining lemons, then place on a rimmed baking sheet and set aside.
In a small saucepan over medium-low heat, heat Coca-Cola until it comes to a boil. Remove from heat and sprinkle over gelatin, then whisk until completely dissolved.
Add lemon juice and remaining ingredients, whisking to combine; transfer to a large measuring cup. Pour mixture into hollowed-out lemons and refrigerate until set, about 1 hour. Slice the lemons in half again and serve.