To make syrup:
Combine sugar and water in a small saucepan. Bring to a boil, simmer for 3 minutes, strain, and let cool.
To make cocktail:
Combine ingredients in a mixing glass. Stir ingredients together, strain into a rocks glass with a large ice cube. Garnish with with a grapefruit swath expressed and inserted into the drink.
Expressing a swath of citrus means taking a thin slice of the peel and bending/pressing it over the glass to pop the citrus rind's pores and unleash the essential oils.