Any time you hear someone want a “ceiling”, understand they are proposing that as the minimum. A 20% GDP cap means that spending will always be at least 20%, and then tricks can be used to keep spending beyond the cap by calling things emergencies or calling expenditures “not spending”. Since the GDP includes government spending, this is the GDP spending ponzi scheme.
Other than World War II, the Reagan Military buildup + tax cuts era, and the Obama years, the government spending has not been above 20%.