GDP and employment are not necessarily related. In fact, recently, we have had things called "jobless recoveries" where GDP goes up but employment does not.
The reason this can happen is that GDP is the value of all final goods and services produced in a country in a given period. If GDP goes up, the value of these things has gone up. But that can happen without more jobs being created.
For example, imagine an economy that has very little in the way of machines. If more machines are invented, GDP will go up as things can be produced more quickly. At the same time, employment may well go down because people will be thrown out of work by the machines.
So -- it's defined as economic growth because the economy is making more things (in dollar value). But that does not have to mean that employment goes up.