How to convert fiat currency to crypto through api

I want to accept the payment in ETH but I also want that if the user don’t have ETH they can also do payment of a fiat currency by their credit or debit card but I get the amount in ETH so, is it possible

You can only receive fiat currency or to a bank account but you would have to set up a Advance checkout in order to receive the payment

Can I use it as fiat to cryptocurrency payment gateway ?

If you can receive money through fiat you should be able to get you crypto currency as a payment get away