https://www.hinduismfacts.org/hindu-scriptures-and-holy-books/manusmriti/chapter-xii/
- All those (doctrines), differing from the (Veda), which spring up and (soon) perish, are worthless and false, because they are of modern date.
Since the Bhagavad Gita is later than the Veda and denigrates "the flowery language" of the Vedas and karma-kanda - would Manu smriti consider such scriptures to be false?