Combine all the ingredients in a liter-sized plastic bottle and chill between –10°C and –5°C (14°F to 23°F). (For a G&T solution that will keep indefinitely, leave the lime juice out in this step.) Using a carbonation rig, carbonate at 42 psi. If omitted from the first step, add the lime juice as the drink is poured into a chilled flute.