I think technically in that example it'd be an extra word, but I get what you are saying...it's all about how pervasive it is in our everyday language, and gonna has YEARS on Imma.

I have a strange relationship with gonna, because it actually DOES annoy me, but I think it's because as a kid whenever I said "gonna" I had some teacher making me repeat it with "going to" all pronounced properly...and god forbid if I should ever write it. :P