We do not know if the gangrene is developing in the hip or it is a gangrene, which is ascending from the leg to the hip. There is a difference between these two, the former could be a result of a local infection where as latter may be the ischemic gangrene of the leg which has gone in to an uncontrollable stage.
In any event the gangrene near the hip is a serious symptom of the under laying disease, the condition needs to treated with a sense of urgency and care, and not taken lightly, as at times it may be a harbinger of bad prognosis. The most important factor to be taken into consideration is the investigation and treatment must not only focus on the gangrene itself but the systemic that have precipitated the condition.