Combine the simple syrup, salt, lime, and lemon juice in a cocktail shaker filled with ice. Shake well, then pour the mixture into a medium saucepan and sprinkle with three envelopes of gelatin.
Allow the gelatin to soak for 2 minutes. Then, heat over very low heat, stirring constantly, until the gelatin is fully dissolved (for about 5 minutes).
Remove from heat and add the gin, stirring well to blend. Divide the mixutre into 2 bowls, then color each with red and blue dye. (I use the natural food dyes available online and at specialty stores like Whole Foods).
Pour into two very lightly greased loaf pans and chill until fully set, several hours or overnight.
Slice the red and blue jello loaves into small squares, then carefully mix them in a lightly greased 9 x 13 inch baking pan OR lightly greased silicone ice cube trays (my preference!) by spooning some of each color into each square.
In a separate bowl, sprinkle the remaining 2 envelopes of gelatin into 1/2 cup cold water. Allow the gelatin to soak for 2 minutes, then add 1 1/2 cup boiling water and stir until dissolved.
Add the can of condensed milk. Stir and allow to cool to room temperature but NOT set.
Pour cooled milk mixture over jello and chill overnight. If you used a baking pan, carefully turn out the jello, cut into blocks, and serve.
If you used ice trays, carefully turn out the jello but skip the cutting part!