Okay I think we all agree that a dystopian future where a cabal of payment processors squelches any dissenting opinions is bad in many ways.
But that's not what's happening here. Stripe has a very specific rule that gab.ai agreed to when they signed up. That is, no porn. Every company that uses Stripe that I've found includes some blurb in their ToS referencing "No Porn Allowed." I've also found it very difficult to find porn on these platforms using some generic search terms.
Of course gab.ai wouldn't want to do include this in their ToS, since they bill themselves as "champions free speech, [and] individual liberty". So now when Stripe comes to their front door asking about that thing they agreed to, now they are whipping up everyone into a moral outrage of "we are losing our free speech!"
Don't fall for it. Yes, those who control the purse strings control the speech, and yes payment processors who allow porn are very very limited and outrageously expensive. But from my experience in webhosting, this is very much tied to the fraud associated with the domain. If they don't like it, they can integrate with a payment gateway and merchant account themselves instead of using a full stack payment processor.