What is the mortality rate of colitis once gangrene sets in?

Depends. Mortality depends on how quickly the patient is taken to surgery after the bowel becomes gangrenous, and on how healthy the patient was prior to getting into the serious bowel trouble (eg. Pre-existing serious liver or heart problems).
Very high. Gangrene of the colon is a very serious complication, with a very high mortality rate. Fortunately, this is a very rare complication; usually due to arterial problems, uncommon to be related to colitis.