The shooter in the Gilroy incident bought his gun in Nevada because he was a resident of Nevada.
While federal law has allowed the sale of rifles and shotguns to non-residents since 1986, the law still requires that the sale be legal both in the state the sale is made and the state in which the buyer lives. It is not legal for a dealer to sell a handgun to a person who is not a resident of the state in which the sale it made.
There is a low-level civil war underway; just not the one the author writes about. The real one is much more important.
Incidentally, it’s worth noting that the three shooters (Gilroy, El Paso, and Dayton) did not share an ideology.