In India various sects exist with people being known as Shaivas, Vaishnavaites or some other sect. It is necessary to clear up the confusion of how these terms are related.
The Supreme God is Sri Krishna who appears as a Consciousness Trimurti of Brahma, Vishnu and Shiva as the Creator, the Preserver and the Destroyer/Procreator of life.
The Krishna in the Mahabharatta and the Bhagavad Gita was not an incarnation of Vishnu, but represented an incarnation of Sri Krishna directly. I say ‘represented an incarnation’ because in reality God does not Himself transform/materialise in a magical way from an invisible Being into a human being for that would be impossible: He creates life through natural forces, that is Nature, and does His work of influencing mankind through the manipulation of those natural forces in real time. The Mahabharatta and its constituent part the Bhagavad Gita were stories written rather than real-life events, but written by an avatar of Sri Krishna (we do not know who this person was). An avatar is a human being who is the beneficiary of guiding revelations from God in the form of thoughts and visions.