Is the Supreme Court the final authority for all federal court cases and interpretation of all federal laws? Yes.
Is the Supreme Court the final arbiter on political disputes about the constitutionality of a law. ABSOLUTELY NOT! NO! NEVER! Nothing in the Constitution forbids political dissent or political activism against a Supreme Court opinion declaring a law constitutional or not. Nothing forbids Congress from going to political war with the Supremes if the black robes go too far into the poitical arena (indeed we see just this happening in nominations through the Senate). Indeed there is nothing preventing Congress from adding fifty new justices to the court, as FDR tried to do. It would be perfectly legal and constitutional. Congress could also cut the funds for everyone except the Chief Justice--the only judicial officer mandated by Article III. And of course Congress and the states can pass new Amendments to quash court decisions.
The Supreme Court is still just a court of appeals. It simply hears select appeals on certain cases. That's all it does. So if they come out with a stupid ruling that gets the law wrong, none of us have to agree with them. We may be stuck with their ruling in certain judicial cases, but that's the extent of their authority. Indeed we can cooperate to reverse them. It's not easy, but it is a valid check on the third branch's power.
As far as gun control, I would expect the Supremes to intervene against draconian national gun control laws, especially if there turned out to be manifold enforcement difficulties and civil protests (which is where we come in). But if they don't, we are left with civil demonstrations and political action. Including, IMHO, active non-compliance with the law. Maybe it won't be possible to get Congress to change its mind, but I'll bet cash money our pals in Juneau will pass a law forbidding any local or state LEO's from cooperating with any enforcement activities under the new law. They did it with the Patriot Act, after all. Then the situation gets very interesting indeed.