For marinade, combine olive oil, wine, garlic, grapefruit juice, and chopped fresh thyme in small bowl. Pour ½ cup marinade over beef in large shallow baking dish or plastic bag; turn to coat. Cover, or close bag and marinate in refrigerator about 3 hours. Refrigerate remaining marinade.
Remove beef from marinade, discarding marinade. Alternately thread beef, onions, peppers and tomatoes on skewers*. Season, if desired, with salt and black pepper.
Grill, turning once and brushing frequently with reserved marinade, 10 minutes or until desired doneness.
*If using wooden skewers, soak in water at least 30 minutes prior to use.
Notes
If preferred, thread vegetables on skewers separately from beef to ensure that they get done.