Place the watermelon in a high-speed blender and blend until fully liquid.
Pour the watermelon through a fine mesh strainer to remove remaining watermelon pulp.
In a large bowl, whisk to combine the watermelon juice, lime juice, honey, and gin.
In a medium saucepan over medium heat, whisk together the gelatin and one cup prosecco. Cook for 10 minutes, stirring continuously, until gelatin dissolves and mixture becomes foamy and doubles in size. Remove from heat and let sit 5-10 minutes.
Whisk the gelatin mixture into the watermelon mixture along with the remaining prosecco.
Once fully combined, pour the jello shots into a 9×13 inch baking dish and refrigerate for at least 4 hours until set, then cut into 1-inch cubes and serve!