Prepare the Glass:

Fill a highball glass (or any tall glass) with ice cubes to chill.

Mix the Cocktail:

In a cocktail shaker, combine the gin, fresh grapefruit juice, and simple syrup (if using).

Lightly bruise the rosemary sprig by slapping it between your hands to release its oils. Add it to the shaker.

Shake well to combine and infuse the rosemary flavor.

Assemble the Drink:

Strain the mixture into the prepared glass filled with ice.

Top off with tonic water and gently stir to combine.

Garnish:

Garnish with a slice of grapefruit and a sprig of rosemary for an aromatic touch.

Enjoy:

Serve immediately and enjoy the refreshing blend of gin, tonic, grapefruit, and rosemary flavors.