Bhagavat Gita is contained within Bhishma Parva:- 25 to 42 are chapters 1 to 18 of Bhagavat Gita.
Bhishma Parva starts with the narration of the geography of ancient India with its kingdoms, rivers and regions in and around India. Then comes the discourse of the philosophy of ancient India between Krishna and Arjuna, which is famously known as the Bhagavat Gita. After that, the parva deals with the battle of Kurukshetra between the Pandavas and the Kauravas and their allies. Kauravas were fighting under the command of Bhishma, the eldest warrior among the Kurus. He was grandfather to both the Pandavas and the Kauravas. The Pandavas fought the war under the command of Sankha a prince from the Matsya kingdom. After Sanka was slain the prince from Panchala kingdom viz. Dhristadyumna became the commander in chief of the Pandava army. The command of Bhishma lasted for 10 days out of the total 18 day war. In some days the Pandavas had the upper hand while in some other days the Kauravas became victorious. Bhishma was a ferocious commander. Pandavas could not overcome Bhishma after trying all their best. Finally they sought help of Bhishma himself. Bhishma gave a hint that he won't fight with the Panchala warrior Shikhandi who was of mixed gender. On the tenth day Arjuna placed Sikhandin in front of him while fighting with Bhishma. Seeing that Shikhandi was in front of him, Bhishma stopped shooting arrows. Arjuna's arrows from behind Sikhandin pierced all over the body of Bhishma and he fell from his chariot onto the battle field on a bed of arrows.