Which test double would you use for a payment gateway: a stub, a mock or a fake?

It depends on what the test needs, and often you use more than one across a suite. Unit tests should never call the real gateway.

Use a stub when you need a canned answer to drive the code: "the gateway approves" or "the gateway declines". It has no logic and no verification, and it is the simplest option.

Use a mock or spy when the behavior to check is the outgoing call itself, for example that the charge was sent once with the right amount and an idempotency key.

Use a fake when many tests need gateway-like behavior with state: an in-memory gateway that records charges, declines a specific test card and returns the same result for a repeated idempotency key. It costs more to write but keeps tests readable.

A dummy is an object you pass only to fill a parameter, such as a logger the test never reads.

Pick the simplest double that answers the test's question, and cover the real gateway sandbox with a few integration tests.