It's restricted and not possible to export the passwords from Gigya to SFTP, while, you can Import Hashed passwords to Gigya from SFTP.
Please refer to Export from Gigya to SFTP.
It should be possible to export the hashed password of the user using the accounts.search REST API method. That said, users can exist in the database with a null password value (e.g. social login, federated login, or user could have been created without a password via API, etc.).
I would recommend validating that the test data has a password first before trying to export it using search, by calling the accounts.getAccountInfo REST API method with the "include" parameter set to "profile, data, password" to check that the account(s) you are testing with actually have a password.
If you still have issues with this please do not hesitate to submit an incident through the SAP ONE Support Launchpad to the "Customer Data Cloud" product or "CEC-PRO" component for further assistance.