As we all know, Sanjaya was narrating the Kurukshetra war to Dritarashtra. How exactly was he seeing the war and how he got that power?
In Mahabharata—a story of war between the Pandavas and the Kauravas—the blind king Dhritarashtra is the father of the principals of the Kaurava side. Sanjaya is Dhritarashtra's advisor and also his charioteer. Sanjaya—who has the gift of seeing events at a distance almost 80 KM of the length(divya-drishti), granted by the sage Vyasa—narrates to Dhritarshtra the action in the climactic battle of Kurukshetra, which includes the Bhagavad Gita.
Sanjaya had the unpleasant duty of breaking the news of the death of Dhritarashtra's hundred sons at the hands of Bhima at different points of time in the battle, and offers the sorrowing king solace in his darkest hours. He is known to be brutally frank in his recital of the day's battle events and his own opinions, which usually would predict the utter destruction of the Kauravas at the hands of Arjuna and Krishna.
In the Bhagavad Gita, passages often start with the Sanskrit words "Sanjaya uvāca:" ("Sanjaya said:"). The entire Bhagavad Gita is Sanjay's recital to Dhritarashtra of the conversation between Arjuna and Krishna.
In verse 18.75 of Bhagavad Gita, Sanjaya categorically accepts that it is by the mercy of Vyasa, that he could directly hear the message of Gita in the battle field of Kurukshetra, sitting in Hastinapura.