To set up a mock payment gateway in a new org, an administrator shouldtake the following steps:

Create a Payment Provider: This is a custom metadata type that defines the configuration and behavior of a payment gateway. A payment provider specifies the name, type, class, and endpoint of the payment gateway.

Create a Named Credential: This is a Salesforce feature that securely stores authentication information for connecting to external services. A named credential specifies the URL, identity type, authentication protocol, and certificate of the payment gateway.

Create a Payment Gateway: This is a custom object that represents an instance of a payment provider. A payment gateway references a payment provider and a named credential, and defines additional settings such as currency, mode, and timeout. References: Salesforce Payments and Order Management, [Named Credentials]