Perhaps it would be more accurate to say that the people living in Qatar are the wealthiest, but GDP per capita is as useful a tool as aggregate GDP by nation.

But if go by total GDP, Qatar is below the Philippines, and I think we can all agree that Qatar in perceptively a wealthier country. The per capita measure matters, once purchasing parity is taken into account, and that measure puts Qatar on top. The fact that it is concentrated amongst a minority of people living hear (nationals and expats) makes it even more apparent. Add then to the lack of spending on defense, little need for social welfare (the poor are workers who leave families in their own country), etc. then you end up with a rather large concentration of wealthy people with a great deal of discretionary income.

My point is that this high concentration of wealth is what makes Qatar agreeable for Western expats as opposed to a place like say Nigeria (much higher national GDP). Whether or not it is the best place, however, is up for debate.