The best way to measure a GDP is by what is supplied on the market. Basically this means that the GDP should be measured by what and how much the economy is producing. The produced goods and services may be only for the country's market, or also for export, but those are the ones that bring in the money in the economy.
If the GDP was measured by what is demanded than we will have a very unrealistic picture. The reason is that the demand for numerous things can be there on the market, bu the financial power of the people may not actually correspond with it, nor the strength of the economy. A perfect example for something like this is Greece.
