GANGCHA is a kind of CAPTCHA that is used on gang websites to prevent outsiders from accesing the website's content (eg. police officers). Usually a GANGCHA is a question about gang's internal affairs that only gang members should be aware of.
Yo dawg, I'm tryin' to get to our website but I can't remember how many hoes we raped last month and the GANGCHA asks me 'bout it. Got any clue?