Google Checkout doesn't require a person to have a merchant account in order to process. Checkout is not a payment method; it accepts and processes existing payment methods, such as Visa and MasterCard.
Since the Google checkout is integrated with Adwords, chances are you will be spending more money than with another processor / IPSP (internet payment service provider). It acts like a gateway but there are subtle differences. Whereas most gateways do not charge a discount rate, Google charges a discount rate and a transaction fee. Other differences can include the limitations, restrictions and reserves applied to checkout accounts. There is also a monthly dollar limit that might be applied to merchants and merchants might process hundreds of dollars before funds are released. And Google might require a merchant to have a payout reserve before they receive any funds.