GDP is good metric to measure industrial output of a country. However for any sort economic activity, which does not include exchanging money, it does not work. A homestead farmer, who is mainly self sufficient, besides selling some goods on the market, is in a much better position then a clothes factory working earning the same amount of money. Both would contribute about the same to GDP. Another one would be the very simple fact that having a natural disaster usually increases GDP a year later or so, due to increased spending on rebuilding. That clearly is not a sign of economic strength of the country though.