I have gotten most of this information from "The complete book of essential oils & aromatherapy" by Valerie Ann Worwood

Gingivitis:

Diluted in 1 teaspoon brandy

3 drops Thyme

2 drops Eucalyptus

3 drops Chamomile

3 drops Peppermint

This is added into water (1 teaspoon into a glass of warm water) other essential oils that can be sued (tea tree, myrrh, bergamot)

Halitosis

added into 1 teaspoon of brandy and mixed then added to a tumble of warm water

caused by digestive problems

2 drops Peppermint

2 drops Lemon

caused by gum disease

2 drops tea tree

2 drop thyme

General

4 drops Lavender

Another mouthwash

2 drops of essential oil of myrrh to 1 tablespoon of vodka .  Take two drops of this mixture and add to a gass of water to rinse your mouth

For mouth Ulcers

Diluted in 2 teaspoons of brandy  (1 teaspoon of mixture into glass of warm water)

2 drops Peppermint

4 drops Lemon

2 drops Geranium

2 drops Thyme