Rapper, in this article, means one who performs rap music. In most cases, this is the definition of rapper, although you could call someone who raps on doors rappers, too. I think the only time I've actually heard that definition used is in "The Raven," though.
Gangsta = gangster.
Pimp -- I think that what this author is saying is that a lot of rappers want to *pretend* to be pimps or gangsters, because pimps and gangsters, in hard-core street life, are cool, have money, and have all the women. At least that's the image given to them by some. It's mostly just silliness.
In the early 1970's, there were some movies with pimps in them, who were dressed in crazy bright clothes and huge hats and platform boots (boots with very high heels). They wore furs and lots of gold, and have become sort of a joke. But those pimps in the movies had a lot of women and acted cool, and so now, that's where the word "pimp" can be used synonymously with "cool" or "sexy," but it's done in a funny way. Most people don't *really* want to find customers for hookers, but they want to be cool and sexy. Here's a photo of "Undercover Brother" who's kinda pimp:
If you ever watch Conan O'Brian's show, then you'll see him sometimes do a skit about being a cool pimp, where he has a fur coat, fur hat, fur-covered car, fur-covered cell phone, etc. And he plays it off as cool and sexy but it's really a joke.
Jon is a different way of spelling John -- but it isn't plural. I *think* that the plural form of John when used as a definition of a customer of a prostitute, is simply "johns."