Veterans' Sentinel Act This bill requires the Department of Veterans Affairs (VA) to implement a pilot program establishing a working group in the Office of Mental Health and Suicide Prevention to collect and analyze data regarding on-campus (i.e., on VA property) suicides and on-campus attempted suicides by veterans. The working group is authorized to implement improvements to the process by which the VA collects data related to on-campus suicides to unify the disparate sources of such data. The VA must annually brief Congress regarding the progress and findings of the working group. The bill requires the VA to (1) annually evaluate the statistical trends of suicides and attempted suicides by veterans that occur on VA property, and (2) determine recommendations for preventing such suicides and attempted suicides. Finally, the bill requires the Government Accountability Office to submit a report on (1) the actions the Office of Mental Health and Suicide Prevention has taken to meet the priority objective of the VA with respect to preventing veteran suicide, (2) the challenges the office has faced in making such actions, and (3) recommendations for improvements.