Grant Reporting Efficiency and Agreements Transparency Act of 2018 or the GREAT Act This bill requires the Office of Management and Budget, jointly with the executive department that issues the most federal grant awards in a calendar year, to: (1) establish government-wide data standards for information reported by grant recipients, (2) issue guidance directing federal agencies to apply those standards, and (3) require the data reported by grant recipients and collected from all agencies to be published on a single public portal.