Gangrene is caused by a bacterial infection that thrives in anaerobic (no oxygen) conditions. Just as you would find in the gut if there is dead tissue, or deep in a limb wound that has an occluded blood supply. Once it gets going, it can be very hard to stop it from spreading, and treatment is (I believe, I may be wrong) often with high levels of oxygen into the tissue.
It's a classic example of the sort of serious complication that shouldn't ever get the chance to set in, if the correct diagnosis and treatment is followed in the first place. And sadly, also the sort of thing that CD sufferers can (at worst) end up with a terminal case of - you never die of Crohn's, it's always the complications...
Charlie and his family certainly have my best wishes winging to them.
Please let us know the outcome.