I think he essentially got corrupted by power, despite looking a "good guy" in TNG. He probably was, but then he changed due to holding command of the Council.
Gowron was never portrayed this way in TNG. Re-watch "Redemption" in which the character was introduced. Everybody considered him capable of poisoning K'mpec.
From the very start, Gowron was always portrayed as the lesser evil compared to the Duras family. He was a son of bitch, but at least he was "our" son of a bitch.