Step 1In a small heavy saucepan, sprinkle the gelatin over the water. Set aside to give the gelatin time to absorb the water, about 2 minutes. Place the pan over low heat and cook, stirring gently with a rubber spatula, until the gelatin is dissolved. Remove from heat and place the gelatin mixture in a bowl.
Step 2To the bowl, gently stir in the gin, then the tonic water and the lime juice. Pour half of the mixture into a chilled gelatin mold and refrigerate until thickened, 20 to 30 minutes depending on the thickness of the gelatin layer and type of mold. Gently add the mint sprigs and several blueberries for decoration, suspending them in the gelatin to decorate that layer. Sprinkle the lime zest over the top. Continue to refrigerate until the gelatin is set but not firm.
Step 3Place 2/3 cup blueberries in with the remaining gelatin and sugar and muddle using a fork until the berries have released their juice and the gelatin is tinted purple (for more coloring and flavor, blend the berries with the gelatin mixture in a food processor before straining). Strain the mixture.
Step 4When the first layer of gelatin is set, pour over the second layer and refrigerate until it is thickened, then decorate with additional blueberries if desired, suspending them in the thickened purple gelatin. Refrigerate until the mold is firm before unmolding.
Step 5To serve, unmold the gelatin onto a serving plate and garnish with additional mint sprigs and blueberries. Cut the mold into slices to serve.