2. To make the citrus relish, combine the grapefruit and orange sections in a mixing bowl. Add the lemon juice and tarragon and set aside. (This can be made in advance and stored in the refrigerator for up to a day or two. The relish should be served at room temperature.)
3. Brush the chicken breasts with oil and season generously with salt and pepper.
4. Grill the chicken for 5 to 6 minutes on each side, turning only once. (Thin breasts will take less time, thicker breasts will take more.)
5. Place the chicken on warmed plates, top with citrus relish and serve immediately.