Add the gin, rosemary simple syrup, and lime juice to a glass and stir.
Fill with ice, add the tonic water, and stir again.
Garnish with a lime wheel and sprig of fresh rosemary.
Notes
Flavor tip: To add some extra flavor, release the oils from the rosemary sprig garnish by smacking it between the palms of your hands, then use it to give the drink a quick stir before serving.