My understanding was that the “first wave”, so to speak, of Gab users were the hardcore folks who genuinely had extremist views.

Free Speech and censorship resistant platforms tend to attract users in waves. The first wave is the most extreme, as those users get kicked off mainstream platforms first. Then subsequent waves of users join as less extreme views become problematic. While those waves are less radical, they still have to contend with being in the same pool as the others. Ordinary folks join for idealistic reasons and then see a bunch of radical content. I’m sure that puts a ton of people off. Parler, it seems, wants to be free speech but not that free speech, if you get what I’m saying.