Step 1Place the gin in a measuring cup. Add the gelatin one sheet at a time, breaking the sheet into pieces if necessary to thoroughly submerge it in the gin. Cover the measuring cup with plastic wrap and allow the gelatin to soften, 5 to 10 minutes.
Step 2Pour the gelatin and gin into a small nonreactive saucepan and heat over low heat. Stir the gelatin until it is dissolved and completely incorporated into the gin. Remove from the heat.
Step 3Gently pour in the tonic water, being careful that it does not bubble up when combined with the gin mixture. Swirl the contents carefully a couple of times to combine, and then immediately pour them into one-half-ounce molds. Immediately place the molds in the refrigerator to allow the gelatin to completely set, preferably overnight.
Step 4To serve, unmold the gelee onto a serving tray. Gently sprinkle each cocktail with a little lime zest. Sift together the citric acid, baking soda and powdered sugar into a small bowl, and sprinkle a little of the combined powder over each of the cocktails. Serve immediately.