Sure, but you can't really prep for that. You can vote, you can campaign for your cause, you can try to educate others...

Well, you can certainly stockpile guns and ammo, but if future legislation makes that illegal... I guess you can risk felony charges and bury your guns in the backyard for when the Germans come, but you're not going to be able to carry them or use them in any practical situation without ending up in prison.

Plus, you know... gun rights activists may win a battle or two, but they are losing the war. The reality is that gun ownership is stagnant or going down - as a consequence of increased urbanization, large-scale immigration from countries with no culture of gun ownership (including most of Europe), and the negative portrayal of firearms in the political discourse and in the movies. Today, something like ~80% of the population is OK with "common-sense gun safety" proposals such as UBC; I'm guessing that something like 60% would be OK with ideas such as permits / background checks for buying ammo (about to become a reality in California), reinstating AWB, or banning open / concealed carry. And roughly 30-40% of the electorate sympathizes with just getting rid of guns altogether, a position also privately held by many prominent Democrats.

Even if Trump wins, unless the gun community comes up with approaches better than impotent rage and political attack ads, the outlook is not gonna be good. Maybe you'll get to keep your AR-15 for another 4 years, but your children or grandchildren will be living in a different world.