In large skillet over medium high heat, brown ground beef; drain and return to skillet. Stir in green peppers, onion, and beef gravy. Simmer until peppers are tender, about 15 minutes. Add tomatoes; simmer 5 minutes longer. Remove from heat and add white wine vinegar and rice. Stir until blended. Slice pita pockets in 1/2. Spoon ground beef mixture into each half of pita and serve.