“Can extreme gingivitis be life threatening?”

No, very rare

Gingivitis is caused by germs, and these germs cause ulcers to form in the gums. Blood can escape through these ulcers causing bleeding gums and mouth odor, but moreimportantly the germs can penetrate through the same ulcers, get into the blood stream and set up shop anywhere in the body. I've personally known three people who developed brain infections from oral germs, one died, and another person who developed a heart infection. Maybe flossing and getting teeth cleaned regularly is more important than you realized?

Severe gingivitis and in turn periodontitis can lead to serious systemic involvement, and in some situations where a patient is already medically compromised, like any long standing infection, can be life threatening. Please seek out professional assistance as soon as possible. Good luck.
Dr. Cyril Tahtadjian