I'm not entirely sure as I haven't fully explored all the details of Anime Another Sub Story, but usually there would be a protagonist who drives the story forward. There might also be a sidekick character who supports the main character.
Well, the Gangsta manga ends with a mix of closure and ambiguity. Some characters get the resolutions they deserve, but there are still elements that make you think and wonder about the future.
It could mean that there is a fan - created story (fanfic) about Naruto where the characters are depicted in a 'gangsta' - like style. For example, they might have the attitude, appearance, or behavior associated with gangsters in a modern or unique setting within the Naruto universe.
Probably not as popular as the more traditional Naruto fanfics. The Naruto series has a large following for its original ninja - based story, and the 'gangsta' theme might be a bit too far - fetched for some fans.