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. 
Red or swollen gums
Gums which bleed easily
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 to determine the extent of the disease.
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. More advanced cases will require descaling to remove tartar buildup.
If treated early, gingivitis is reversible, however, if it is left to progress to periodontal disease, the damage is irreversible.
Endodontic disease refers to any inflammation of the pulp, known as pulpitis. Pulpitis can be reversible or irreversible. In the case of reversible pulpitis, once the cause of inflammation is removed, the pulp returns to its healthy state. Left untreated, irreversible pulpitis occurs, resulting from severe inflammation of the pulp, which is extremely painful. Over time, the pulp becomes necrotic (dies), and pain subsides.
Periodontal disease (also known as gum disease) is the most common oral disease to affect cats.
Plaque is a sticky biofilm composed mostly of bacteria (predominantly streptococcus) which forms on the teeth. Over time, plaque, saliva, minerals and food debris mineralise, causing tartar (also known as calculus). Tartar is yellow in colour and is found along the gumline, where it meets the teeth. This leads to inflammation of the gums.
At this stage, proper treatment can reverse the problem. Left untreated the tartar begins to collect under the gum line. Toxins produced by the bacteria in the plaque can irritate the gums, which in turn stimulates an inflammatory response, it is a combination of toxins released by the bacteria, and the inflammatory response which causes the destruction of the supportive structures (gingiva, alveolar bone, cementum and periodontal ligament). Gums separate from the teeth, forming pockets (spaces between the teeth and gums) that become infected.
Advanced dental disease, FORL and tooth fractures all introduce bacteria to the roots which can result in a tooth abscess.
Reluctance to eat
Bleeding from the nose
Left untreated the infection may spread to other parts of the oral cavity, possibly causing a drainage wound on the face
Tooth extraction and flushing of the affected area and antibiotics.
Feline ondoclastic resorptive lesions (FORL)
Also known as resorptive lesions, feline resorptive lesions, neck lesions, cavities, cervical line lesions and invasive resorptions, these painful lesions are one of the most common dental problems in cats. There are three types of lesion;
Internal resorptive lesions
External ondoclastic resorption
Cervical line erosions
It is estimated that 20 to 67% of cats have one more of these lesions. 
Lesions usually begin under the gingival margin and are caused by odontoclasts which are cells whose role is to absorb the bone and roots of deciduous (baby) teeth. In the case of FORL, these cells reabsorb the adult teeth.  Lesions typically occur under the gum line, making early identification difficult. Premolars are most often affected. Your cat will display extreme sensitivity if these lesions are touched.
Reluctance to eat
Cherry red gums
Visual examination of the teeth and the use of a dental probe.
Treatment depends on the severity. There are 5 classifications for lesions with stage 1 being the mildest and stage 5 the most severe.
Your veterinarian will apply a fluoride varnish or sealant on the teeth
Moderate to severe:
Extraction of the affected tooth, or teeth.
There is currently no way to prevent FORLs as veterinarians do not know what causes it.
Also known as lymphocytic-plasmacytic gingivitis-stomatitis-pharyngitis (GSPC), stomatitis is a common disease-causing chronic inflammation and ulceration of the soft tissues in the mouth. There is no definitive cause but it is thought to be multifactorial with an immune-mediated component, possibly representing a hypersensitivity to oral bacterial antigens.  Other possible factors include oral irritants, some viruses, immunodeficiency diseases, metabolic diseases, drug reactions etc.
Reluctance to eat
Gums which bleed easily
A thorough oral examination can diagnose stomatitis. Diagnostic tests may be nessary, which include:
An oral biopsy may be performed to determine if the lesions are caused by other diseases such as neoplasia (cancer) or eosinophilic granuloma complex. Biopsy should reveal a dense infiltration of lymphocytes and plasma cells.
An x-ray to check the condition of the dental roots and bones. Stomatitis often affects the molars and pre-molars more than the canines and incisors.
Stomatitis is very difficult to treat and response to many treatments are poor. If the cause can be identified, then specific therapy to manage or treat the cause.
Professional cleaning of the teeth under anesthesia is necessary, as periodontal disease may cause or at least contribute to stomatitis.
Antibiotics given long term may be of benefit.
Cats unresponsive to treatment may require extraction of all teeth behind the canines to provide long-term relief, while this may sound extreme, your cat will get along just fine without these teeth with the assistance of a softer diet.
Clean your cat’s teeth daily which will help to keep plaque under control.
 Cat Health Encyclopedia – Edited by Lowell B. Ackerman.