Sanjaya names the source of his extraordinary access. It was Vyasa's grace that made this possible. Without the sage's gift of divine sight, Sanjaya — sitting far from the battlefield — could never have heard Krishna's words as they were being spoken.
There is gratitude in every syllable. Sanjaya does not take credit for what he received. He attributes it entirely to Vyasa's kindness. And what did he receive? Not a second-hand summary. Not a later retelling. He heard the supreme secret of yoga directly from Krishna — Yogeshvara, the Lord of Yoga Himself — as the words left His lips.
The word 'sakshaat' — directly, firsthand — underscores the immediacy. Sanjaya did not learn the Gita from a book or from someone else's account. He was a living witness, hearing it in real time through an extraordinary gift.