I’m gonna order a pizza;. Gonna, like the pronunciation it denotes, only occurs when going to is a modal verb indicating a future tense (something that is bound to happen or is planned), and not for lexical uses.
Gonna = going to (for future actions) wanna = want to (for intentions and desires) gotta = got.. The meaning of gonna is —used for 'going to' in informal speech and in representations of such speech.