Keep a dated copy of the official program page and the identity and family information needed for the initial application. After payment of the nonrefundable processing fee, follow the USCIS email instructions for Form I-140G and its supporting documents; the official FAQ does not publish a universal civil-document list or a lawful-source requirement for the gift and fee.
Prepare identity, relationship and funds evidence, then wait for instructions
The official FAQ says that after the initial application and processing fee, USCIS emails instructions to complete Form I-140G with supporting documents; USCIS and the Department of State may request additional information. It does not publish a universal list of civil, police or lawful-source documents, so those should not be stated as Gold Card requirements in advance.
Keep a dated copy of the official page with the figures as they appear on the day, retain payment records, and respond to the instructions actually issued. A household may preserve other transaction records for its own financial and tax advice, but that prudential step should not be attributed to the Gold Card FAQ.