You gauge the future market by looking at the past and current trends. Even the classic "GP's" do not bring any money what so ever. Cars go up in value due to people's want for them. If you look at the trend in the collector car world it's the cars guys(and gals) wanted when thy were in High School or one they can relive their past in. Seriously, who wanted a GP in high school? Not because of the price, but take price out of the equation and see if it's still desirable. I have a feeling the majority would pass and go for a sports car not a sports sedan.
Lets take for example the 97 GP. It's already 15 years old and the price used for them even in good to excellent condition is still dropping. If a car hasn't bottomed out in price by this time it's hard to believe it will ever go up in value.
Dont get me wrong by my post I love my GP and it's worth more to me than what most are willling to pay. I'll more than likely keep mine unless someone offers me a crazy amout of money or has a trade that would be stupid to pass up. I personally can't wait to get a new daily and retire this one to weekend duty.