Bring the hibiscus flowers, sugar and 1/2 cup water to a boil in a small saucepan. Reduce the heat to medium and simmer, stirring, until the sugar is dissolved, about 2 minutes. Transfer to a small bowl and refrigerate until chilled, at least 1 hour or overnight.
Strain the hibiscus syrup into a liquid measuring cup, reserving some of the flowers for garnish. Combine the gin, lemon juice and 1/4 cup of the hibiscus syrup in a small pitcher. Stir in the tonic. Add more hibiscus syrup to taste.
Fill 4 glasses with ice. Pour in the cocktail and garnish with the reserved hibiscus flowers.
Photograph by Linda Xiao