Mastercard and Visa don’t habitually keep people from using their networks to make payments. They may stop businesses from accepting payments.
Besides, the “true believers” are willing to send money orders and get other funding.
People these days with unpopular views lack imagination. The entire civil rights movement thrived against a much more hostile environment than Gab. No one is burning down their houses, lynching them etc
The entire legal (at least in states where they are legal) weed industry has a viable business without banks, credit cards, etc.
If you truly believe in your message and enough other people are interested, you don’t let minor roadblocks stand in your way.
The civil rights movement didn’t boo hoo that things were “hard to work around”.
I'm quite confident that the civil rights movement did complain that things were hard to work around. That didn't stop the movement of course. Just like people are still fighting the big tech monopoly, despite the uphill battle