In all my years in Tokyo, I have never before found a cool bar in Gotanda. Sure, I've happened on a few in and around Osaki and Meguro, the stops on either side on the Yamanote Line loop -- but never one in Gotanda itself.
One reason for this may be that a mini red-light district was the first form of nocturnal entertainment to get a foothold there, making it a destination that most casual drinkers have avoided.
Then came Happy Jack.