If a part of the body is gangrenous, it has gangrene: the flesh there is dying or already dead because blood is not flowing to it. So this is the describing word, and gangrene is the name of the condition. A doctor might say a toe has become gangrenous — that toe now has this rotting, blood-starved damage. It is a hospital word, quite formal. You almost only use it about living tissue. People sometimes stretch it in a dark joke for anything “rotten”, but that is loose talk; the careful meaning stays medical. Same picture as gangrene; this form just lets you put the idea in front of a noun.

壞疽的