Hi gangrene suggests that the tissue is dead. There is no point in trying to save it as it is already dead. It will also serve as a source of infection and cause the infection to spread further into her leg and we surely don't want that. There is a procedure called debridement where the dead tissue is removed and the surrounding healthy tissue is encouraged to grow with daily dressings and a skin flap if necessary. But this is possible only if the gangrene is superficial. If it has gone deep into the muscle layers or even the bone your mother is going to benefit by salvaging the gangrene toe. Make a wise decision. Take care :)