If the goal of this bill is to limit the government's long-term debt without tying its hands during times of economic emergency, wouldn't a GDP cap somewhat defeat that purpose, since GDP will necessarily shrink during a recession?
I see what you mean. If, during a recession, the government deems it wise to stimulate the economy with additional expenditures, a GDP cap can indeed be counterproductive. Indeed, in times of a crisis the GDP is lower. As a consequence, the government cannot spend as much money (in raw numbers) as in a situation where the GDP is higher.
I am not a fan of massive government intervention even during a recession, but for someone who wants the government to take drastic measures in times of economic hardship your point is very valid.