The Lord Himself has also established Himself as the Supreme Personality of Godhead in the Bhagavad-gītā. He is accepted so in the Brahmā-saṁhitā and all Purāṇas, especially in the Bhāgavata Purāṇa, kṛṣṇas tu bhagavān svayam (SB 1.3.28). So therefore we should take Bhagavad-gītā as it is directed by the Personality of Godhead Himself.