In Shropshire (and possibly elsewhere in the Midlands), there's an old saying along the lines of:
Mar 26 2006, 6:45 AM
"You canna say munna cos that just inna right, and you munna say wunna cos it inna polite".
I guess the same applies to "gonna".
Generally, any word with recognised meaning is a "real word", but that doesn't mean its use will be accepted in all circumstances.
On the whole, "gonna" is a widely accepted contraction in the spoken (depending on whom you are speaking to) or texted English but not very accepted at all in written English.