A Warning about CBO Scoring
Recall that the bill raises taxes substantially. Some of these tax hikes are the explicit tax increases on capital income to pay for the insurance subsidies. Some of these tax hikes are the implicit marginal rate increases from the phase-out of the insurance subsidies as a person's income rises. Both of these would be expected to reduce GDP growth.
Indeed, to be very wonkish about it, these tax changes could have especially large GDP effects. Some people like to argue that taxes have small GDP effects because income and substitution effects offset each other. But if you give someone a subsidy and then phase it out, both the income and substitution effects work in the direction of reducing work effort.
Why does CBO assume no change in GDP? It is not because the CBO staffers necessarily believe that result. Rather, it is just one of the conventions of budget scoring. Their estimates should come with a warning label: