The percentage tax revenue plays on the U.S. gross domestic product is actually quite low, however. They're actually a rather small portion. Only two other developed nations (Mexico and Chile) have lower tax ratios.
Another point in argument against these GOP claims and tax policy suggestions: the US is the only nation on the planet that taxes persons who live under its own definition of poverty.
For an interesting take on these figures, see the writeup on PoliticusUSA.