Best cure for acute necrotizing ulcerative gingivities?

Proper Oral Hygiene. You definitely need to see a dentist. Depending on the severity you may need an extensive type of cleaning and debridement of your gums. That must be followed up with proper brushing and flossing to achieve proper oral health.
A series of things. Antibiotics coupled with a professional dental cleaning will be the first things to be done at the dental office along with exceptionally good home care on your part. If this is inadequate to address the issue, gum surgery is sometimes indicated to try to get things back to a state of health.
Professional care. A good cleaning, perhaps after the area is numbed, and a course of antibiotics. Please see your dentist.