It would never go to the Supreme Court because it would NEVER be ratified into law by 3/4 of the states.
He's playing a bigger game-- probably aiming at the Electoral College. He knows this BS amendment will never happen, but he will use that and portray it as an unfair "failure" of the Constitution where many low-population states can block this kind of idiocy that urban areas want. They can convince urban idiot mouth-breathers that "it's the guns". His bigger game is to work towards the removal of states rights and make Federal laws and bureaucracies the ultimate authority (just like State laws and buraucracies like CARB in Commiefornia). He wants totalitarianism.
Eliminating the Electoral College would still require a constitutional amendment, so it would get no more traction than an amendment to repeal the 2nd Amendment. There is simply no way that 38 states will approve an amendment that effectively makes a minority of states the arbiter of all federal elections.
The only leeway is that the Constitution does allow states to choose how they allocate their electors, i.e. winner-take-all or proportional allocation. Several blue states have taken a stand against the "tyranny" of the Electoral College by passing the ridiculous National Popular Vote Interstate Compact, which says that they'll give all their electoral votes to whoever wins the national popular vote (regardless of the popular vote in their own state) once sufficient states join the Compact. Sounds great, right? No more pesky Electoral College! One person, one vote for President! Democracy wins!
What none of them point out is the the Compact isn't worth the paper it's written on. All it will take is for the shoe to be on the other foot, i.e. a Democrat squeaks by with more electoral votes than popular votes, and every blue state will repeal it so quickly that everyone's head will spin. It's pure political show.
Newsom is preparing for a presidential run in case Biden is incapable of serving a second term. No one is going to amend the Constitution, and Newsom knows it. He's just pandering to the gun control lobby for future votes.