Put the 1 1/3 cups water and sugar into a wide, thick-bottomed saucepan and bring to the boil. Let boil for 5 minutes, take off the heat, add the lemon zest, and leave to steep for 15 minutes. Strain into a measuring cup, then add the lemon juice, the tonic water, and the gin; you should have reached the 6-cup mark; if not, add more tonic water, gin, or lemon juice to taste.
Soak the gelatin leaves in a dish of cold water for 5 minutes to soften. Meanwhile, put the ¼ cup of water into a small saucepan and bring to the boil. Remove from the heat, squeeze out the gelatin leaves, and whisk them in. Pour some of the gin and lemon syrup mixture into the saucepan and then pour everything back into the measuring cup. Pour into the mold and, when cold, put in the refrigerator to set. This should take about 6 hours.
When you are ready to unmold, half-fill a sink with warm water and stand the mold in it for 30 seconds or so. Clamp a big flat plate over the gelatin and invert to unmold, shaking it as you do so. If it doesn’t work, stand it in the warm water for another half minute or so and try again. If you’ve used a dome mold, surround the gelatin with white currants, or fill the hole with them if you’ve used a ring mold. Raspberries are just as good, but dust these with confectioners’ sugar—it sounds fancy, but it makes the pale-jade glimmer of the jelly and the otherwise-too-vibrant red of the fruit come together on the plate. The white currants should be left to glimmer, opal-like, without interference.
To make a vodka and lime jelly, simply substitute 6 limes for the 2 lemons and use vodka in place of the gin.