You need to provide an Identification document; typically your passport, while appearing for the GMAT test. The type of ID documents accepted may vary from one country to another, and you need to check the specific requirements for the test centre in your country.
The ID document should be current, valid and legible, failing which you will not be allowed to sit for the exam. In such a case, you will also have to forfeit your exam fee and pay the full fee if you decide to appear for the exam again.