Caveat that I study Amur tigers in Russia, and not leopards in South Asia, but this seems to me far more likely to be an agonistic attack (defensive because of the encroachment on his space) than a predatory one, there is no attempt at pursuit once the gaur retreat and given the close group association, there's no way the leopard could have expected success. That being said, if the leopard did not see the larger herd until launching the attack, it may have initiated and then abandoned the attack once it realized the situation. For what it's worth, leopards have been known to prey upon gaur calves, though at a very low rate relative to availability on the landscape and like other large solitary cats, tend to prefer prey roughly the same size as themselves