It depends on what taxes you include.
This is all taxes: individual and corporate income, social insurance, and excise. Note that the tax rate has hovered just above 20%.
You can find graphs of average tax rate, by the type of tax, at: US Federal Taxes by Income Level. They were derived from data published by the Congressional Budget OFfice -- which is also archived on that page.
Tax rate is not the same as tax revenue.
Revenues, not rates.
5% of GDP would be about right if we have a legitimate government.