Registration as a business entity is an absolute necessity if you want to avoid any future problems. Paying taxes is just one part of it. Even though no separate law exists for crypto payment gateway because US law classifies cryptos as stocks/assets and not real money. So there are many open questions yet to be answered by the respective agencies. But when you are building a payment gateway for cryptos, it is a business, so you should not avoid the registration part. If you can hire a lawyer to get those open dots connected, that will bring you some good insights. But paying taxes is not enough.