What testing approach to apply when there is no documentation - black box, white box, or gray box?

In the absence of documentation, the most suitable testing approach will be black-box testing.

This means that the tester considers the system as a "black box" — without access to internal code or architecture, they verify functionality based on input data and expected outputs.

Why black box:

• No documentation — meaning no description of internal logic, so white-box (white-box) testing is impossible or difficult.
• Grey-box testing assumes partial knowledge of the internals, which is absent in this case.

During the process, you can:

• Study the system's behavior through interfaces.
• Use exploratory testing to identify main scenarios and bugs.
• Gradually create documentation based on observations.

Thus, black-box testing is an optimal starting point for testing without documentation.