Official transcripts or certified replacement documents directly from the issuing institution are the strongest option, along with any records showing an institution's name change, merger, or closure if that's part of the gap. Personal copies or informal summaries generally aren't sufficient substitutes.
Request records directly from the source
Institutions typically have a formal process for issuing official transcripts or replacement credentials, and that process should be started as early as possible given it can take time. If the original institution no longer exists under its former name, ask whether a successor institution or an archive holds the records, and get documentation of that institutional history alongside the academic records themselves. Hypothetical example: an agriculturist trained abroad holds a degree certificate but no transcript, and the university requires a request submitted through a local agent.
A first review would start that request in the same week the gap is identified, ask the employer to hold the start date rather than set one, and in parallel obtain whatever the applicant does hold so that the shape of the gap is known. It would also ask the institution to send the documents directly rather than through the applicant, since a sealed transmission from the issuing body answers a question that a forwarded copy leaves open.