IPaymentGateway is only for payment gateways where the card is charged directly without the user leaving your web site (ie currently Authorize.NET and PlugNPay are our only IPaymentGateways).
PayPal and Google checkout are special cases that are not based on IPaymentGateway. It is possible to implement other card processing gateways by implementing IPaymentGateway, but we don't currently have any easy way to plugin other external payment systems that work similar to PayPal or google.
Hope that helps,
Joe