Trump will very quickly move on beyond the GST and get on to other anti-trade measures in the Canadian economy, measures that have been mentioned by anti-trade elements in the US in the past.

Like national health care, which is obviously an unfair subsidy to business- and an enormous competitive advantage to Canadian companies, that don't have to pay for health care for their workers.  It'll have to go. 

And French labeling requirements.  

And the use of French in magazines and newspapers.  

It's not very likely that we'll get any kind of a replacement for NAFTA.