Matt if you read through the OP of that thread, I kind of give a ideal definition of what the GP refers to. There is no one answer to who they are. The best that can be said is, everybody has their groups, if you are not in said group, you are considered GP. Well, in our case, if you are not a theme park or coaster enthusiast, you are more likely part of the GP.
I'd say the GP is just anyone who is not a theme park or roller coaster enthusiast. That's really the only requirement. If you casually enjoy roller coasters and visiting parks, you're apart of the GP. It's not a bad thing. Sure, we joke about the GP asking if this ride goes upside down (a very real occurrence, by the way), but it shouldn't really be derogatory.
I mostly think of the GP as the families who visit when I reference them in a post, but it's just a term to describe non-enthusiasts, or more accurately, normal people.
There is a specific GP thread that CK created, because I expressed distaste for the term and the way it tends to be used. I still dislike "GP" and all of the implications thta seem to go along with it.