I agree with everyone else regarding the ‘gonna’ issue. I also agree about contractions, that it does depends on the character but contractions are usually necessary to keep the dialogue from becoming stilted. If your characters are in a formal situation like a courtroom, a historical context, the army etc, then they would use proper English but in every other situation they’d use contractions because that’s what’s normal.
Also, what your professor may have meant was that there was too much chatter not relevant to moving the plot forward. It’s good to have a casualness to the dialogue if the characters concerned are casual sorts of people (which would be most people, teens or adults) but if it’s not moving the plot forward or telling the reader something new about the characters then it needs to be cut. The last thing you want to do is waste valuable space you need for your plot.
Hope this helps.