Most notably, the possibility of making it more difficult to get imported guns. If your preferred manufacturer doesn't have a factory in the country, it could potentially be more difficult/more expensive/impossible to get one of their guns.
IMHO that is unlikely to happen because of free trade issues, the 2A notwithstanding.
Most of the countries on America's "Not Good" list- China, Russia, Iran, the DPRK, etc.- already cannot legally export guns to the USA, or are limited to a very short and specific list of models (e.g. China and Russia).
The countries that currently export firearms to the USA in large numbers are all countries with which the USA has friendly trade relations: Germany, Austria, Italy, Belgium, Brazil, the Czech Republic, Canada, Switzerland, even Japan. If the administration tries to shut down the import pipeline from these countries, their trade representatives are likely to squeal very
loudly, and may retaliate against American exports in other sectors of the economy- during a time when the administration is trying to present itself as a friend of U.S. export manufacturers!
Furthermore, such a move would probably have little effect on the overall volume of gun sales, because the 68 GCA has already shut most foreign makers- with the possible exception of Taurus- out of the high-volume lower end of the market. American gunmakers have that territory staked out now.
The only scenario under which I could foresee a large firearms import prohibition would be a generalized return to trade protectionism, but I don't see that happening in the current political climate, because of its potential to destabilize global trade at a time of uncertainty.