Short answers, pulled from the story.
The Indo-Greek king Agathocles issued coinage bearing images of deities related to Vaisnava imagery around 180 BCE. These coins display Sañkarshana-Balarama with a mace and plow alongside Vāsudeva-Krishna holding a conch and wheel.
The Heliodorus Pillar in Besnagar stands as a stone monument dated between 125 and 100 BCE. It bears an inscription by Heliodorus who was an ambassador from the Greek king Antialcidas that dedicated it to Vāsudeva.
Two Puranas dominate later storytelling regarding Krishna: the Bhagavata Purana with twelve books totaling between 16,000 and 18,000 verses and the Vishnu Purana. The tenth book of the Bhagavata Purana alone holds about 4,000 verses dedicated entirely to Krishna legends.
Vallabha Acharya established the Pushtimarg sect promoting pure monism called Shuddhadvaita regarding Krishna as Supreme Deity. This tradition emerged during the medieval period alongside other theological schools like Vishishtadvaita and Dvaita.
Devotional movements devoted to Krishna became prominent in southern India during the 7th to 9th centuries CE. Alvar saints of Tamil Nadu produced the Divya Prabandham collection including Andal's Tiruppavai songs conceived as gopi perspectives.
By 1965 Bhaktivedanta Swami Prabhupada traveled from West Bengal to New York City spreading Krishna-bhakti movement outside India. He formed International Society for Krishna Consciousness ISKCON one year later sharing Gaudiya Vaishnava philosophy globally.