Fill a cocktail shaker with ice. Add the grapefruit juice, tequila, agave liqueur and simple syrup. Stir just to blend, then strain into two rocks glasses filled with ice. Serve garnished with strips of grapefruit zest.
Combine the sugar and 1 cup water in a small saucepan. Bring to a boil over medium-high heat, stirring occasionally. Once the sugar has dissolved, turn off the heat and add the basil. Allow the syrup to cool completely before removing the basil. Cover and refrigerate until ready to use. Leftover syrup will keep for up to a month.