Stripe has pretty strict rules over prohibited businesses (https://stripe.com/us/prohibited-businesses). For example, here's their list of "high risk businesses", prohibited by their financial partners:
> Bankruptcy lawyers; computer technical support; psychic services; travel reservation services and clubs; airlines; cruises; timeshares; prepaid phone cards, phone services, and cell phones; telemarketing, telecommunications equipment and telephone sales; drop shipping; forwarding brokers; negative response marketing; credit card and identity theft protection; the use of credit to pay for lending services; any businesses that we believe poses elevated financial risk, legal liability, or violates card network or bank policies; any business or organization that a. engages in, encourages, promotes or celebrates unlawful violence or physical harm to persons or property, or b. engages in, encourages, promotes or celebrates unlawful violence toward any group based on race, religion, disability, gender, sexual orientation, national origin, or any other immutable characteristic
"Travel reservation services and clubs" bit me at some point, I wanted to build a flight reservation service and was surprised to hear I couldn't. But apparently, it's a high risk business because lots of people chargeback flight tickets.
Also, Gab completely fails that last criteria, they're not kidding anyone.