Yes for both, in different ways.
How do I test FPS and EPS?
You do not need Government Gateway credentials of your own. Switch is_test_mode_enabled on in the employer's RTI submission settings, and submissions go to HMRC's test service carrying our test credentials. That takes FPS and EPS end to end with no Gateway set-up on your side. You register your own credentials only when the employer moves to production; gov_gateway_credentials_set on those settings confirms they were registered.
Remember to switch test mode off for a live employer. With it on, real submissions do not reach HMRC's live service.
How do I test DPS notices?
HMRC sends notices through its Data Provisioning Service, and as far as we know there is no test channel that delivers one, so a notice cannot arrive in the sandbox the way a real one does. Instead, we place test notices directly on your sandbox employer, on request. Contact us with the sandbox employer and the notice types you want (P6, P9, SL1, SL2, PGL1).
You can then test everything your integration touches: GET .../dps/ to list notices, GET .../dps/{id}/ to read one, GET .../dps/by-sequence/{sequence_number}/ to look one up, and POST .../dps/{id}/apply/ to apply it to the employee. Applying sets a new tax code for a P6 or P9, or starts or stops a loan for SL1, SL2 or PGL1, from the next payrun calculated for that employee. Applying is refused if the notice has already been applied or arrived with errors, so you can test the failure paths too. Notices are read-only over the API, which is why seeding has to come from us.
Can I see the results in a user interface?
Yes, the sandbox front end is at https://payroll-sandbox.moonworkers.co.uk/auth/login.