More complex means that you focus on what you can do best and pay others to do what they can do best. Instead of growing your own wheat and pottery, you and others pay a third person to get a tractor and you can instead focus on doing pottery and sell them and buy wheat. This way more gets produced.

And while the wheat you grow yourself isnt part of the GDP, the wheat the third person grows for you, is. Therefore a more complex economy significantly boosts the GDP more than increased productivity. So if you produce your own wheat and your own pottery and your neighbor does the same, the GDP is 0. If you sell your pottery and your neighbor his wheat, both get added to the GDP.