“Brahman spoke through Krishna by entering into him like a ghost.” How do we respond to this idea?

From: Keshav
How to deal with statements like ” Krishna did not spoke Bhagavad Gita , it was someone else ” ?

Now a days in many T.V shows and social media it is being said by scholar’s and guru’s that Krishna did not spoke Bhagavad Gita , it was someone else who spoke within him .

they say the original brahman who got inside Krishna like ghost and spoke through him .
These people are claiming that Krishna was not god , the spirit inside him was the real god, and the word ” God and Maam ( Me ) ” in bhagavad gita refers to this spirit .

(This answer is long because it analyzes key verses chapter-by-chapter from the first to the eighteenth chapter of the Gita. As this is a common doubt, i have answered at length so as to address it systematically based on the Gita itself)