I've only known one but it was not a good hunting dog.
It belonged to a friend that I usually hunt with 1 or 2 times per year. In the times I hunted with it, it never once pointed or retrieved a bird. It generally ran away and disappeared as soon as it got loose. Not like it was hunting or seeking birds it was just running. We would then waste time looking for and calling the dog until it finally came back. Then it would be satisfied or tired out and walk behind us for the remainder of the hunt. One time it took off as we were trying to load it up at his house. We wasted over an hour looking for and chasing it all over the neighborhood. If it was up to me we would have just carried on with the hunts and let the dog come back (or not) on its own. But he was always worried and we'd hunt for the dog.
The dog also had no interest in birds. Even when we walked it into a spot where a bird had gone up or another dog was working a bird it showed no interest.
Lastly it was nasty. It would snarl and snap at people if it didn't like what it was being told. If it was on a couch/ bed/car seat and you tried to move it or you tried to get back food it snatched you'd get snapped at.
The dog was poorly trained and spoiled by it's owner but it just was not a natural hunter.
I got so sick of that dog that the owner didn't get asked along on hunts for a few years until the dog passed on.
That's only one dog and I'm sure there are other Gordon's that are great. But that's the only one I had any first hand experience with. It was a good looking dog though.