In about 200 verses, as a dialogue with his client Arjuna in the first part of Bhagavad Gita, Krishna discusses the subject of karma threadbare in the battlefield of Kurukshetra. Even if you don’t believe this happened and you don’t believe Krishna is god incarnate as billions of Hindus have believed, no matter.
Just read the Bhagavad Gita and make up your own mind.
In the case of Buddha, Jesus and Mohamed, founders of great religions, we seem to have historical evidence that they were men who walked the earth. This has not prevented us from giving them divine status. Believers revere Buddha as god and worship him in temples. Knowing how Buddha denied the existence of any god, he is probably turning in his grave.
Jesus became the son of god for no fault of his, perhaps because he addressed what is beyond as Father. Billions worship him too, knowing fully well he was human. The fact that no one can worship a form in Islam gives Mohamed respite.
In the case of Jehovah, Krishna , Shiva and other such immortals in Hinduism and Judaism, the jury is out.
They too were probably humans who walked the earth. They probably were historical characters, and not mythological creations. Yet, over time, people created legends around them to make them divine. Words attributed to them have become scriptures.
Why not, if people believe and follow words uttered by men in other religions as scriptural?
Unlike other Hindu scriptures such as the Vedas, Bhagavad Gita is not a collection of prayers, incantations and chants. It is not stories and parables as in the Bible. It is not commandments of life as in the Koran. Somewhat like the Tao and Dhammapada, Bhagavad Gita is about life solutions. Its verses address our inner energy. Krishna seeks to raise our inner energy to his level.
In about 700 verses, Krishna advises Arjuna how to lead his life so that his Past karma, the prarabda karma, is eliminated. This is the essence of Gita.
Tomes have written commenting on these 700 simple verses. I do not need to add to them.
One of the best versions i have read is by Eknath Easwaran, in 3 slim volumes, published by his Blue Mountain Press. Do read him.