Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry it has taken so long to answer your question. The most common cause for this is boredom and/or anxiety. This anxiety could be due to territorial issues with your other dogs or just Gabby's personality.
Take Gabby out for many walks and take some time to play with her more than the others. When you are gone, give Gabby a long lasting toy such as a Kong available at the pet store. I would also recommend that you purchase a DAP Collar. This is a canine pheromone that acts as a calming agent decreasing any anxiety and is available at the pet store.
Most often, this is enough to stop this obsessive behavior.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta