It sounds like she has a gum infection - probably due to bad teeth or abscesses. I would take her to another vet for a second opinion as they should be able to get this under control with a good dental cleaning and a round of antibiotics. I would check with a feline only vet as they can find treatments that general practitioners often miss, and if your girl is loosing weight because of it, they need to find a treatment that is effective.
In the meantime, I would put her on a soft food diet - canned or pouched as dry food must be very painful for her right now.
Please let me know if I can help further.
I agree with my colleague that a feline vet should examine your girl. I'm also wondering, was your cat diagnosed with Lymphocytic Plasmacytic Gingivitis? This is a poorly understood inflammatory oral condition that some cats get. Basically, they become allergic to the plaque on the teeth.
The cause is unknown. Cats with FELV or FIV are prone to this, but most of the cats who have it are negative for those viruses. Various viruses and bacteria have been suspected as causes but never proved. There are other conditions that may appear similar, such as eosinophilac granuloma complex, or cancerous lesions. To be certain of the diagnosis, a biopsy can be performed. Bloodwork can be helpful also in diagnosing.
Treatment for a mild case includes keeping the teeth very clean (since the plaque is causing the allergic response), and/or various combinations of antibiotic and steroids, and pain medication for flare ups. However, this disease can be hard to control with medication, and it may get worse. Once it progresses, the only treatment clinically shown to really help this condition is extraction of all the teeth behind the fangs. A lot of cats will get relief from this measure, although it's not a guarantee either. The good news is, cats do great, absolutely fine without those teeth. The cats who respond to extraction therapy are very happy cats without those teeth because they have no more mouth pain.
I have a link here with more information:
I would seek a feline vet, make sure your cat is FELV and FIV negative, and discuss the best course of action for her.Here is a link to the American Association of Feline Practitioners. You can search for a cat vet in your area:
I very much hope your girl can be helped. Please feel free to ask any other questions you may have.
As an afterthought - this could also be feline stomatitis - which is fairly common. I'm attaching a link for you that may help!