Gangrene is read as a neglected problem spreading until it threatens more than where it began. Dream books see it as a strong image of something that needs to be let go.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.