GDP as reported is the product of a particular well-defined calculation. That product can easily be calculated to whatever precision you feel like.
When you say "it couldn't be measured with such precision", how do you define the Gross Domestic Product that couldn't be measured precisely?
I'm assuming that the GDP is some sort of measure of the health of the economy-- that's why people are concerned with it. The health of the economy seems to me like rather an approximate sort of thing.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.