gonna
part.part. a short way of saying 'going to' when you talk about the future. You use it in casual speech or text messages with friends.
part. a phonetic reduction of the phrase 'going to' when used as a semi-auxiliary to indicate future intent or inevitability. Restricted to informal registers and casual speech; generally avoided in formal writing except when transcribing dialogue.
I'm gonna see a movie tonight.
If we don't leave the house right now, we're gonna miss the start of the show.
While the speaker used formal vocabulary throughout the presentation, his frequent use of 'gonna' created a surprisingly casual atmosphere that put the audience at ease.
Contraction of going to. Attested since 1913 (OED, "gonna"). The pronunciation of present participles with the sound n rather than ng has a long history (see g-dropping on Wikipedia).
Used only before a base verb to indicate the future; it cannot replace 'going to' when 'to' is a preposition (e.g., 'I'm gonna the store' is incorrect).
I'm gonna to goI'm gonna goThe word 'gonna' already includes the 'to' from 'going to', so adding another 'to' is redundant.