There are infectious causes of gingivitis, and stomatitis which can be passed on from one cat to the others in a multi-cat household, like yours. The most common causes are Bartonella disease, Calicivirus,
feline infectious peritonitis, Felv/Fiv, and feline herpes virus.
Bartonella (cat scratch fever) is treated by the antibiotic Azithromax. Herpes outbreaks can be controlled by using Lysine supplementation.