"The GP" is a construct. It's a person or group of persons who only go on coasters to fill up the cattle pens so we have to queue longer, they only go to parks to block your path or spill their ice cream all over you. They aren't exactly uninformed, but so misinformed and fond of telling everybody about it that they drive enthusiasts to madness. They bring their kids just to annoy you. They think they are experts, because they've read the page on amusement parks in an old Guinness' Book and played RCT3 once. They vandalize, urinate, scream, fight and generally pester just to make your potentially nice day miserable. They make theme parks build boring attractions, fountains and drink stalls. They blind the park managers' eyes to all the awesome that can be done with an idle plot of land and some money. They are everything we don't want to be (that's not to say none of us behave like GP).
"The GP" is the manifestation of the sum of all annoyances that happen at theme parks. An idea, more of a description of an actual group of people. You can refer to the GP, but not point them out. You can not point at a person and say "he's part of the GP", but you can wave your arm at a crowd and mumble "GP" through clenched teeth.
Of course we hate the GP. But if asked "Exactly who are the GP?", we have no answer. All that we know, is that they're not us.