Do gangster-themed Madoka subtitles really exist?
So, after a while of researching this, it appears that someone had taken a subtitle file and run it through Gizoogle's Translation site. 

Gizoogle is a parody of Google based off of Snoop Dogg's lingo 

What this does is translates sentences such as:


  Madoka and Homura are best witch friends forever. 


into Gangsta slang:


  Madoka n' Homura is dopest witch playaz forever n' shit. 


The subtitles have not been published online, whoever created them merely posted screenshots. But if you had an original subtitle file, you could replicate it easily enough by using the same process.