Shopping cart local testing with dummy payment gateway?

I’d like to set up an isolated version of ERPNext for training purposes that will simulate an online store, but that will be in a classroom lab that is not connected to the Internet.

My understanding is that there is no way to simulate a payment gateway - you must use a real payment gateway like Stripe, even if you are just using it test mode. Is this correct?

Can I create a custom app that injects a new “Dumpay Settings” into the list of “Payment Gateways” when it is installed? How do I set that up?

The Payments app stores gateway definitions in “payments/payments/payment_gateways/doctype/_settings” folder.

In developer mode, I can go into the Payment Gateways list and create a new “Dumpay Settings” document and it creates it in the Payments app in my build hierarchy.

I currently have the following directory structure in my apps folder:

You can see that there is a “dumpay/dumpay/payment_gateways/doctype/dumpay_settings”. My intuition behind doing it this way is to create an app that can define a Dumpay Settings doctype that can be used by the Payments module without actually modifying the Payments app.

Of course, I see no evidence that there is a Dumpay Settings doctype - and don’t really know how to create one.

I’ve looked at the source code to the Payment Gateways, but they are all configured in the Payments app. Can I configure a Payment Gateway that will play with the shopping cart without altering the Payments application?