If my mom has gangrene on her left toe, does this mean that it is on other parts of her body?

Not necessarily. If she has diabetes or peripheral artery disease, the toes are very susceptible to gangrene. They are at end of blood supply and nerves, and prone to injury. Depending on cause, the same pathology may involve all vessels and nerves, though she is not yet seeing symptoms in those areas. Treatment of underlying cause can slow progression.
No. Gangrene means death of tissue related to lack of blood flow. Having severe peripheral arterial disease (pad) can increase the risk of gangrene of a toe, and makes the other toes susceptible to gangrene, but doesn't necessarily mean there is gangrene elsewhere at the moment.
No. Needs to be checked for pad as well as heart disease and cerebrovascular disease. Is she diabetic? Does she smoke?