What GNP Means on a Montana Vacation

A few years back, my family took a vacation trip to the Canadian Rockies and Banff National Park. We took the train from Minneapolis out to northern Montana: turns out that they have a "family car" that sleeps five. Then we rented a car and started exploring.  After about an hour on the road, I noticed a bumper sticker on a car with Montana license plates that said "GNP," inside a circle. I kept looking, and soon spotted others.

I began wondering why GNP was on bumper stickers in Montana. Of course, I knew that the U.S. government had shifted over from emphasizing Gross National Product to emphasizing Gross Domestic Product a couple of decades ago. But was there something about the economy of the state of Montana that would make GNP a more attractive choice? I don't know much about Montana's state economic issues. It has a lot of mining, right? Is there some reason why the presence of mining companies based outside the state might mean that there is a divergence between GNP and GDP in a way that would matter to the state of Montana?

I couldn't figure out any obvious answer, and so I worried about these bumper stickers on and off for a couple of days, as we hiked around Glacier National Park. And then when picking up some maps of hiking trails in gift shop, I realized that in Montana, GNP is Glacier National Park. So I had to buy the hat:

For previous episodes of when I or others have been unable to leave economics behind on vacation, see this post about tasting high-end olive oil and this post about hiking in Yosemite.