Gingivitis is a general term for inflammation of the gums (gingiva). It may be localised to one tooth or may be widespread affecting numerous teeth.
Gingivitis is the mildest form of periodontal disease. Infection and inflammation spread from the gums to the ligaments and bone that support the teeth. Left untreated, loss of support causes the teeth to become loose and eventually fall out. 
Unhealthy teeth and gums have a greater impact on the body than just causing bad breath, pain, and infection. As the gums have a rich blood supply, bacteria is readily transported to other organs (such as the liver, kidneys etc.) in the body causing damage and even organ failure.
Dental disease: Gingivitis is caused by a build-up of plaque (bacteria and food debris). In the early stages, plaque forms on the teeth, if it isn't removed, it hardens and becomes tartar (also known as calculus). Tartar is yellow in colour and is seen along the gumline, where it meets the teeth.
Plasmacytic-Lymphocytic Stomatitis (LPGS): This is a severe form of gingivitis-causing extreme pain. The cause is still unknown. It appears to be a hyperactive immune response. Feline calicivirus, FIV, and FeLV have all been implicated.
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Bad breath (halitosis).
Red or swollen gums, especially along the gum line.
Gums which bleed easily, especially when touched.
Receding gum line.
Difficulty and or reluctance to eat.
Your veterinarian will perform an examination of your cat's mouth for signs of gingivitis such as a build up of tartar, red and inflamed gums, bad breath.
Full mouth x-rays may be recommended to determine the extent of the disease.
Plasmacytic-Lymphocytic Stomatitis: A biopsy is required to diagnose this condition.
Your veterinarian may also wish to do an FIV and FeLV test to rule out these two diseases as a cause of gingivitis or plasmacytic-lymphocytic stomatitis.
This depends on how far advanced the gingivitis is. Early cases of gingivitis which haven't progressed far may possibly be treated at home with regular dental cleaning. Some treatments your vet may perform include:
De-scaling to remove tartar build up.
Cats are expert at masking discomfort and pain and many pet owners may not notice that their cat has a problem. This is another important reason why regular, annual check-ups with the veterinarian are so important. Even if you believe your cat is in good health, a thorough physical may uncover a problem in the early stages. If addressed immediately, gingivitis is reversible, if it is left to progress to periodontal disease, the damage is irreversible.
Regular descaling by your veterinarian.
Diligent home dental care in the form of regular brushing of your cat's teeth.
Anti-inflammatory drugs such as prednisone.
Interferon and other immune modulators.
If these treatments don't work, the only option is to extract the affected teeth.
There are several ways to prevent gingivitis. The key is to prevent tartar build up in the first place.
Regularly brushing your cat's teeth. This will need to be done with a special cat toothbrush and toothpaste. Never use human toothpaste on animals.
You can purchase special diets which are designed to reduce plaque and tartar formation. One such food is Hills T/D which can be purchased through your veterinarian.
Feed raw chicken necks or bones. This is a somewhat controversial topic. In Australia it is quite commonly recommended as a way to reduce plaque and tartar formation, however, there are risks associated with feeding raw bones to cats. Speak to your veterinarian for his/her opinion on feeding raw chicken necks and or bones.
 Medline Plus