Dear Mr. Goldstein,
Thank you for posting your request on History Hub!
We are unaware of any comprehensive lists compiled by the Secretary of War of all Civil War veterans or any Department of Defense lists of all of the veterans alive at any given time period.
Should you be interested in researching the 75th Anniversary Gettysburg reunion, we searched the National Archives Catalog and located Correspondence, Reports, Lists, Photographs, and Blueprints of the Federal Commission of the Commemoration of the 75th Anniversary of the Battle of Gettysburg, 1936 - 1938 in the Records of the Office of the Quartermaster General (Record Group 92). For more information about these non-digitized records, please contact the National Archives at College Park - Textual Reference (RDT2) via email at Archives2reference@nara.gov.
We also located (Three) Photographic Albums [of the U.S. Signal Corps and (the) Army Air Forces Activities] Commemorating the 75th Anniversary of the Battle of Gettysburg (in Gettysburg, PA), 7/1/1938 - 7/4/1938 in Record Group 92. For more information about these non-digitized records, please contact the National Archives at College Park - Still Pictures (RDSS) at firstname.lastname@example.org.
Next, located Photographs of American Military Activities, ca. 1918 - ca. 1981 in the Records of the Office of the Chief Signal Officer (Record Group 111) that includes the digitized Photograph of Union and Confederate Veterans Shaking Hands Across the Stone Wall at the 1938 "Blue and Gray Reunion" at Gettysburg. It is available using the Catalog.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT2 and RDSS. We apologize for this inconvenience and appreciate your understanding and patience.
Additionally, the Gettysburg National Military Park Pennsylvania Library & Research Center and the Gettysburg College Special Collections & College Archives may have collections pertaining to the 75th anniversary activities, as these two institutions were both involved in the events.
We hope this is helpful. Best of luck with your research!