I would continue to communicate with the Shelter manager. Show him/her the proof that the dog is yours. Ask them how it can be resolved. Let them know that you are considering going to the media to plead your case. Let them know that if they won't contact the new owners, you will have no other choice but to contact the local newspaper or other media in hopes that the new owners will see the story (or friends of the new owners will). Don't bad mouth the Shelter if you're interviewed. Explain how the dog was lost and how hard you searched for Gabbie and how badly you want her back.