In a large saucepan, place the sugar and water and bring to a boil, cook for about 5 minutes or until the sugar has dissolved. Set aside and allow to cool.

In a large bowl, combine the cooled sugar syrup, tonic water, gin, and the zest and juice of the citrus.

Using a stand mixer or handheld mixer, whip the egg white until soft peaks form, about 5 - 8 minutes.

Gently fold the whipped egg white into the sorbet mixture until just blended.

If using an ice cream maker, pour in the sorbet mixture and freeze according to manufacturers' directions, about 40 - 60 minutes. If using the no-churn method, pour into a container, such as a loaf pan, covering with plastic wrap, and freeze for about 2 hours, stirring up the mixture every 30 - 45 minutes to break up ice crystals with a fork.

Transfer the sorbet into a freezer-proof container, seal, and freeze for at least an additional 4 hours or more until ready to serve.

Garnish with lemon and lime slices if desired.