The GRPR is fully within the remits of existing trade agreements and does not violate US constitutional law, so as long as the EU is not selectively targeting US companies(and there is no evidence of that being the case), it's going to take a lot of crazy politicians and some tearing up of bilateral trade and cooperation agreements for the American authorities to take any action other then assisting the EU in collecting fines from American companies.
There nothing new about companies enforcing their existing laws on imported goods and services and the eu-us free trade agreement's already contains clauses where both parties have to assist each other if a company is trying to evade those rules. The only thing new is that the regulators are now considering digital services to cross the border when there is money flowing the other way.
If the US were going to raise a stink they would have done so when the EU kept doubling MS antitrust fine until MS eventually paid up, issuing GDPR fines to discord that is smaller then what smaller European companies have been fined ain't going to be an issue for the trans Atlantic trade relationships. Especially as GDPR style rules are being proposed at the state level in the US.
Region locking of copyrighted material is where the real blocks in content is going to come into play(as it already have) but that is a whole different can of mud.