The Bhagavad Gita manuscript is found in the sixth book of the Mahabharata manuscripts – the Bhisma-parvan. Therein, in the third section, the Gita forms chapters 23–40, that is 6.3.23 to 6.3.40.
The Bhagavad Gita is often preserved and studied on its own, as an independent text with its chapters renumbered from 1 to 18.