I have not had experience with gangrene, but the image you posted reminds me of my hens bruises that showed up a few days after she was attacked by a stray small dog. The pictures are below (and yes she survived, full feathered and never got an infection).
Wow, that was a lot of typing. You'll have to learn to upload on here
I saw that on my pheasant's injury. It isn't gangrene, I don't know if that is how they bruise, but mine result in huge chunks of yellow dry skin like stuff that peeled off and resulted in fresh skin underneath. It looks like your wound is healing pretty normally.