K'Mpec was poisoned. I'm pretty sure it was Duras who did it because Duras was a backstabber and capable of anything. Gowron, as hostile as he was to the High Council, doesn't seem like the type to kill in a cowardly way. He was openly a fighter. If he wanted to fight K'mpec, no matter how suicidal that was, he would have done it.
I've seen some people say that it was the Tal Shiar but there were only Klingons on the ship, so it was only Romulans in the form of aids to them (in this case Duras). In fact, the fact that he threatened Worf's mate openly, would suggest that he was openly hostile, and was not the type to do something sleazy like poison someone's drink.