Wet or dry. If it is wet gangrene (infected) it can cause sepsis and death. If it is dry gangrene, it can be monitored closely and you can live with it as long as it doesn't convert to wet gangrene. Gangrene is dead tissue, bacteria loves dead tissue- so dry gangrene is a nidus for infection. Talk with your doctors to help weigh the specific risks, benefits, consequences and overall prognosis.
Many... Unfortunately if there is gangrene and infection the problem is likely to spread higher in the leg. If left untreated the infection can go in the bloodstream. This is very risky and the person may die.