Put sugar and water in a saucepan and bring to the boil while stirring. Let simmer for about 3 - 4 minutes. Stir in between. Let cool down.
Add the lemon and lime juice. This results in the so-called Sweet & Sour Mix. Put it in a cool place for about 2 hours.
Pluck the basil leaves, place in a blender with the Sweet & Sour Mix and chop briefly. Strain through a fine sieve. This mixture can be prepared and, if necessary, placed in a cool place.
To prepare the cocktail, add ice-cold gin and cold sparkling water to the mixture. Pour crushed ice into a chilled highball glass with a capacity of approx. 240 ml and fill up with the Gin Basil Highball Cocktail. Serve decorated with a basil leaf.