Combine water & sugar in a saucepan. Bring to a simmer over medium heat, stirring until the sugar is dissolved. Remove from heat; add the grapefruit zest and lavender. Allow mixture to steep for about 30 minutes then strain into a clean container. Store in the fridge until ready to use.
In a Collins glass filled with ice add 1 ounce of gin, 2 ounces grapefruit-lavender syrup, top with soda water and stir. Garnish with a small slice of grapefruit. Syrup amounts may be adjusted to preferred sweetness.