Will the GOP remain an authoritarian / nationalist party post-Trump ?

It's important to remember that the Republicans were laying the foundations for Fascism well before Trump. If they can continue going full authoritarian after Trump, they will. However, I think that's only possible if they can sell Fascism in the guise of a cult of personality, which means they need another Trump. Thankfully, I don't see any Republicans right now who can fit that role. So they might have no choice but to rebrand, a process that will take decades.

Looks like, from the answers given.

Maybe, what does your crystal ball tell you?

There is no Republican Party. There is only the MAGA Party.

Only dupes and nostalgics think they're supporting the Republican Party.

Fiction processing on repeat can be edited

Most of them were already authoritarian so yes they will. They were just waiting on someone like Trump to make it okay