Krishna is mentioned in many script like Mahabharata, Bhagavatham, Gita Govindam etc... Gita is spelled in many ways and all are same. There are 18 Chapters. You can download PDF format of Gita from website kairaly.net and Bhagawad Gita.
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Gita is definetly written before the time of Budha. Not only Gita, but all other Hindu epics are written before that. None of the Hindu epics ever mntioned about Buddhism, Jainism, Sikhism. All these religions are derived from Hindu religion.