gangster vs felon

gangster and felon both are nouns.

Word	Noun	Adjective	Verb	Adverb
gangster	Yes	No	No	No
felon	Yes	No	No	No

As nouns, felon is a hypernym of gangster; that is, felon is a word with a broader meaning than gangster:

gangster: a criminal who is a member of gang
felon: someone who has committed a crime or has been legally convicted of a crime

Other hypernyms of gangster include criminal, crook, malefactor, outlaw.

gangster (noun)	felon (noun)
a criminal who is a member of gang	someone who has committed a crime or has been legally convicted of a crime
	a purulent infection at the end of a finger or toe in the area surrounding the nail