Answers
The applicant will produce (among others) the papers submitted at his application for the Golden Visa (tax number, contract of your land property, etc.).
Additional information to my previous answer: If you are married to a Greek citizen, you can apply for Greek citizenship after three years of legally living in Greece. Perhaps you heard about this provision mentioned three years.
One of the main conditions for a citizenship application for Golden Visa holders is to prove seven years of continuous and legal residence in Greece. The documentation needed to be applied are, among others, a passport photocopy with all the related entrance and departure dates/stamps, a photocopy of the residence permit title, and photocopies of the latest seven tax declarations.