GDP does not measure wealth. It measures (or attempts to measure) Production of wealth. Very similar, yet different.
If I build a house, that is in GDP. I have produced something which is worth something. If I sell the house, I haven’t added any “production of wealth”, I have merely traded the asset for a different asset. If I was to measure it when sold I would be double-counting it, because I already counted it when I built it.
Now there’s a little bit of a pony there: the real estate commission is ‘production” and any fix-ups I do are probably “production” and if I hire somebody to stand in the front yard with a sign that says “Open House Sunday 2-5” that counts too. But the house itself? Nope.
To positively murder the already dead horse, if I add a bedroom, that’s GDP because I have “produced” something new. When I sell the house with the added bedroom, doesn’t count insofar as GDP/GNP goes..
Likewise, importing (or exporting) a fine art piece isn’t “GDP” because I haven’t produced anything, I’ve just traded something for something. (Ignoring auction house fees, carpenters boxing it up, etc.) Likewise precious metals, jewelry, etc. (The original production of making the fine art piece? Yep. GDP.)
It’s Gross National PRODUCT(ion). See? If we spend on oil cleanup or education, that counts because we’re ‘producing” a service. Military too. If we trade assets (money for gold, etc) that doesn’t count because we haven’t created anything, we’ve just shape shifted.
GDP misses a lot. Unpaid work (housewives), volunteer work, public services like parks & playgrounds which do not charge admission, and so on. GDP measures flow into production. That which doesn’t 1) flow and 2) produce don’t count
In the short time I was an economics major (around ‘66) it was all about GNP. Then it became GDP. Pretty much the same car, different color.