有冇人可以解比我聽??thank you very much

gonna is informal
you can't use it when writing essays (not written english)
you only use it in e-mail, or when chating with friends (not in a presentation or oral assessment)

going to is formal
which means you can use it in formal english, e.g. formal letters, essays...

**but actually gonna has the same meaning with going to

wanna (informal) = want to (formal)

I wanna go home now = I want to go home now