GDP includes busines investment and inventories. Neither are taxed under the NRST.
http://www.colorado.edu/Economics/courses/econ2020/section6/GDP-components.html
You are misinterpreting your link. GDP only includes economic activity, not assets such as inventories. Your link clearly shows that GDP includes CHANGES IN inventory. If businesses push more inventory out the door than it receives, that is an increase in GDP, and a sales tax will capture it.
A flat income tax does not tax inventories or business investment either. That's why I said a sales tax and a flat income tax are economically equivalent.