Maybe it's the locale I'm in, but I'd be surprised if even 15% are GTP's? And it's not like GP's aren't a dime a dozen around here - they're just all SE's or GT's. I could count the number of GTP's I've seen recently on one hand and still have fingers left over.
But I would have long since lost count of lower model GP's. I would argue that it's still among the top three most ubiquitous cars in this area.
(Then again, it used to be a big GM town.)