Collins
I understand that the Collins dictionary defines gangsterism to be a person using the methods and behaviour of gangsters, and the tactics associated with gangsters. But it is my current understanding that you do not actually have to be a member of a gang to use the methods, tactics and behaviours associated with gang members, and thus be commonly referred to as a gangster that commits gangsterism.
the methods or behavior of gangsters 2. the use of tactics associated with gangsters, as intimidation or violence, in order to achieve something
Urban dictionary
Certainly, the Urban dictionary which mostly describes the common usage would appear to define gangsterism as simply "acting" like a gangster. Again, I understand this to mean that you do not have to be a member of a gang to be regarded as a gangster.
Subscribing to a course of action reminiscent of organized crime. Acting or conducting business in a way that is violent and totally without conscience. Acting like a gangster/thug/hoodlum.
Wikipedia
Wikipedia describes a gangster as invariably describing a criminal, and also points out that there is no clear consensus about what constitutes a gang. So here again, I read from this that you do not have to be a member of a gang to be regarded as a gangster.
In modern usage, the term "gang" is generally used for a criminal organization and the term "gangster" invariably describes a criminal.2 Much has been written on the subject of gangs, although there is no clear consensus about what constitutes a gang or what situations lead to gang formation and evolution.
Merriam Webster
Merriam Webster According to the Merriam Webster dictionary, a synonym of gangster simply describes a violent and brutal person.
a violent, brutal person who is often a member of an organized gang
Question
My understanding is that going by the modern and common usage of the word, you do not require to be a member of a gang to be called a gangster. I could however be totally wrong in my thinking and understanding, so therefore I will ask the question, do you need to be a member of a gang to be described as a gangster and accused of gangsterism?