A GP can be a cheap car.
People who own cheap cars tend to fall a little lower on the social ladder.
People who fall a little lower on the social ladder like to take the piss out of each other for fun.
People enjoy this. You get a good laugh. The other person laughs too. Everyone becomes happy.
If not, you're too sensitive or have no sense of humour.
This is the part where I insult you but clearly you can't handle it.
And before anyone gets pissed at me let me just say: If you make over 100 grand a year, you're probably not driving a 97-03 GP.