Geeta is part of epic Mahabharata which is a story written. As per historians, when written initially Mahabharata had only around 10 thousand verses but later it grew as every generation kept on adding episodes and new stories into it and now it has over 1 lakh verses. So Geeta which is part of Mahabharata also got changed tremendously as many different philosophies were introduced into it during the course of time. We can find many philosophies of upanishads in it like samkhya, adwaita, dwaita and people even introduced caste system into it by making it appear as if Krishna is teaching it. Gandhiji was also influenced by this and that is why blindly supported caste system.
So we can not consider Geeta as Revelation. Its a philosophical and mythical book.
Yes, Krishna was not there in Vedas, upanishads. He came to light through Mythology (Puranas). So he is a mythological figure. God taking human form (Avatar) is even against the concepts of earlier Hindu scriptures ie Vedas and Upanishads.
Read this article to understand how and why different philosophies and mythology were mixed as part of bhakti (devotion) movement. That gives you good understanding of how concept of God changed in Hinduism over time.