In speech I'd use it if you mean to say "going to". I'm going get that file... I'm gonna get that file. Mind you, you wouldn't use it if you're saying I'm going to the store... I'm gonna the store. I'd never use it when writing English.

As Dave says, NEVER use it when writing English. It looks horrible. 'Gonna' is what 'going to' sounds like when someone is speaking fast, for example 'I'm going to miss the bus' may sound like 'I'm gonna miss the bus'. It isn't actually a word - it's just a sound of two words run together.

Agree with Dave and Su.Ki about not using "gonna" or "wanna" in written English. You will seem uneducated.

gotta = going to. if you are going to do something ,you can say I'm gotta do something :)