GDP is a reasonably good measure of a nation's economic activity and per Capita GDP is a reasonably good measure of overall well being under all but the most extreme circumstances. Using purchasing power adjusted GDP is a bit better. IMO, using median income is even better. But GDP figures tend to be more accurate and less subject to bias and thus more commonly used.
To address your snide side comment... Gates visiting would not change the per Capita GDP. GDP and per capita GDP is not calculated that way.
GDP = private consumption + gross investment + government spending + (exports − imports)
and
Per Capita GDP = GDP / Population
Visiting rich foreigners, or even rich expats who just reside somewhere but have no business activity do not change GDP except through their local spending (private consumption).
To address your snide side comment... Gates visiting would not change the per Capita GDP. GDP and per capita GDP is not calculated that way.
GDP = private consumption + gross investment + government spending + (exports − imports)
and
Per Capita GDP = GDP / Population
Visiting rich foreigners, or even rich expats who just reside somewhere but have no business activity do not change GDP except through their local spending (private consumption).
Read http://en.wikipedia.org/wiki/GDP for more info.