That's a very impressive one, which I hadn't even considered. But as far as I know, Gorbag never met Ufthak (it's Shagrat who tells the story: "Don't you know anything? This fellow ain't dead). At least, I'm pretty sure that's the case.
Also, I don't think that Gorbag ever met Shelob. For one thing, the Morgul orks seem rather ignorant concerning her, and also he's still alive, right?
I've got a rather different train of thought, one which depended on Sauron (so I don't feel right disapproving the above post), but which doesn't assume that Gorbag and the Nazgul ever met (even though it may seem like a logical assumption).
*Thinks to self: SO should have made it Shagrat and not Gorbag.