Gingrich is a former Speaker of the House who is a trusted Trump surrogate and on the shortlist for Secretary of State and said on Trump's behalf during the campaign that the House should reinstate the House Un-American Activities Committee (or a similar body) for the purpose of interrogating, blacklisting, and deporting Muslims. He's got enormous influence in the Trump administration and with his former colleagues in the House, he's got Republican control of the House, Senate, executive branch and possibly even the Supreme Court soon enough. He doesn't have to be in the House itself to have influence in convincing them to implement Trump's policies.
The very fact that he suggested bringing back something so intimately and negatively linked with Red Scare fearmongering, blacklisting, and legislative corruption and was not only not immediately laughed off the campaign and disqualified from further office but is now in line to be our Secretary of State and in charge of diplomacy of all things should give people on both sides of the aisle pause. Once the Red Scare fervor died down in the late 50s (though HUAC lasted longer and continued in different forms) HUAC's kangaroo court behavior was condemned by politicians on both sides and called the most Un-American thing about the whole situation by Truman, yet we have a major political figure in the President-elects cabinet shortlist proposing it as a serious idea and not immediately being dismissed from consideration. I don't get how anyone, Republican or Democrat or other, can not find that scary as hell.