I think one source is Keith Olbermann on ESPN’s SportsCenter in the mid-1990s, he used it all the time. That era of that show had a lot of influence on certain modes of ironic discourse. If you can track down Olbermann’s book written with Dan Patrick about The Big Show, as they called it, you can actually find written references to it. I think they actually spelled it differently, like Gggguh or something, but it’s the same thing.

I and my friends have been using it for years, in both email and spoken conversation. We always used it as an expression of exasperation, in place of or in conjunction with the likes of “Ack”, “Argh”, and “Goddamit”. We pronounce it like it looks, like “ah” with a “G” (as in “Golly”, not “gerund”) in front.I know it is frequently used on the wonderful televisionwithoutpity.com.And — talk about going way back — I specifically remember an Archie comic book in which Veronica’s father, Mr. Lodge, let loose a terrific “GAH!” about something that pissed him off. That was when I was maybe five years old, so it’s certainly the first time I came across it.All of this, as far as I can remember, predates Bridget Jones.