Can i get gangrene if my gallbladder fails completely? my hida scan shows its only working 15

Yes still possible: Even with a poorly functioning gallbladder due to the increased potential for stasis and abscess.

"Apples and Oranges": The cck-hida scan measures gallbladder emptying; it is used for people who do not have gallstones yet have gallbladder symptoms (biliary dykenisia). While there is a place for gallbladder surgery in this setting, this disease does not cause inflammation in the gallbladder--in fact, under the microscope, the gb looks entirely normal. Therefore, you need not worry about gangrene.

Very unusual: Gangrenous gall bladder is extremely rare and affects predominantly older men with diabetes and cardiovascular disease. If clinical symptoms and hida scan are compatible with diagnosis of gall bladder disease, then surgery may be the answer.