As an English teacher, students often ask me what gonna, wanna and gotta mean. They mean:
gonna = going to
wanna = want to
gotta = got to -> slang for "have to do something"
I could say these words are not English, but the truth is they are used in American English idiomatic conversation everyday. These expressions are known as reductions. Reductions take a few words and compress them into a shorter slang expression. These expressions should be understood in spoken English, and if you live in the United States, they're certainly OK in conversation. However, they are not used in written English.