GDP -- Gross Domestic Product -- basically means the sum of the value (in the economic sense) of all goods produced domestically during a given period, e.g. a year.
If you want to measure the "health of the economy", that's quite different. You'll have to define what do you mean by that expression and then decide which measurements do you want to consider. For example, some people might consider the unemployment rate to be one those measurements, or, say, the Gini index, or the median income, or... the possibilities are endless.
Why do people measure the value of all the goods produced domestically during a year?
If nothing else, there has to be a fudge factor because some of the economy is underground.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.