If the gangrene is dry on the outside and on the inside, if touching the gangrene feels dry and the inside does not feel soft, then one is half way there to getting rid of the gangrene.
When the gangrene is dry inside and outside, that means the clostridia is no longer present. Wash vigorously with hydrogen peroxide. Hydrogen peroxide releases oxygen, and the skin and the gangrene dry out. Continue washing with hydrogen peroxide until the scab comes off and there are no more scabs remaining on the gangrene area, i.e., until new fresh skin appears in the area.