Add the sugar, tarragon and 1 1/2 cups water to a small saucepan. Bring to a simmer, stirring, until the sugar is just dissolved. Remove from the heat and let steep 15 minutes. Strain the syrup and refrigerate until cold.
Add the rose wine, grapefruit seltzer, 1/2 cup simple syrup and a few slices grapefruit to a pitcher (save the remaining simple syrup for another use). Serve immediately or refrigerate until ready to serve.