Gateways are services that can be used to take creditcard payments online, the most popular of which is Paypal.
Here is a screenshot of the gateways being output on the Booking Confirmation page.
It's important to remember that Property Managers will not see the option to choose a payment method when making bookings, only non-manager users will see those options. If you're making test bookngs to ensure the payment functionality is performing correctly, please log out first.