GPTB stands for "Grey Ponytailed Brotherhood". This term was introduced by MO's resident psychotic and entirely inexperienced faux motorcyclist KPaulcook.... now labelled Ksquid. At first the term was supposed to designate Harley riding middleaged bikers but as time went along it expanded to include anyone who likes to ride any kind of motorcycle except a race replica. Anyone who owns or likes Harleys or cruisers and also owns or likes naked standards, dirt bikes or race replicas is also a GPTB.
Perhaps it's easiest to understand GPTB now in a negative sense. The only people who aren't GPTBs are people whose entire riding experience is taking Sunday rides to the Rock Store or Burger Barn on race replicas and then standing around comparing stats and congatulating themselves on how owning a race replica makes them superior as individuals to everyone else. They are usually parked in front of coffee houses and put maybe 50 miles max per week on their bikes. When the weather is perfect. And when they aren't out buying a dinette set.