Your payment confirmation and reference, identity and civil documents for everyone you intend to include, and the evidence your category depends on. Instructions will name what the I-140G filing needs; assembling passports, birth and marriage certificates, and employment or business records now reduces the preparation work left when a response deadline arrives.
Building a document set that survives comparison
Keep names, dates and spellings consistent across passports, certificates and the payment record, because mismatches are what generate questions. USCIS requires full certified English translations of foreign-language evidence; check the separate consular language rules for that stage. Submit what the instructions request rather than filing something thin with the intention of adding to it afterwards; missing documents can lead to a request for evidence or a refusal.
If an official notice contradicts your own records, investigate the conflict before acting on it. Assign each document to the body that issues it and you will see which ones you cannot produce yourself. Passports come from a passport authority, civil certificates from a registry, employment and business records from the organisations that hold them, and certified translations from a translator who attests to their own competence.
Request the slowest of those first. The applicant's own task is narrower than it appears: gather, check that names and dates agree across the whole set, and be ready to respond when instructions name what the I-140G filing requires.