Oddly, over and over, I seem to read that civilians need to carry guns to protect themselves from criminals who carry guns. Now, you're saying that a particular subset of them, gangsters, carrying guns makes it more dangerous only for those gangsters, but not for other civilians? Just how many gangsters do you think there are in the city of Philadelphia, that they dominated a study where well over 700 people were shot? Does every gangster carry a gun, and if not, are the gangsters without guns more or less likely to be shot than those that carry?
I don't know the answer to any of those questions, and they certainly could affect the interpretation of the study. However, I don't see where not having the answers is a good reason to dismiss the study, either. It's only one data point, though.