Shake gin, grapefruit juice, and simple syrup and strain over fresh ice in a highball glass. Top with soda water. Garnish with sliced cucumber and a sprig of rosemary.
Notes
To make rosemary simple syrup, measure equal parts water and sugar. Bring water to a boil and add sugar, stirring constantly until sugar is completely dissolved. Add 2 sprigs fresh rosemary and simmer for 15 minutes. Cool, strain, and store in the fridge.