The format of a payment gateway extension is described in the Create a Credit Card Payment Gateway Integration chapter. For a cash payment gateway, the gateway.json file should be similar to the following:

{
  "provider": "Cash Payments",
  "paymentMethodTypes": ["cash"],
  "transactionTypes": {

    "cash": ["initiate", "cancel"]
  },
}

The extension must also create user interface controls (for example, checkboxes) for merchants to specify the countries that cash payments are supported for.