Sustainable Pension Plan Act of 2019 This bill requires the Government Accountability Office (GAO) to study and report on the pension system for the employees of the government of the U.S. Virgin Islands (known as the Government Employees Retirement System). The GAO study must analyze (1) the benefits provided, including a comparison to those offered to private employees; and (2) whether the system is adequately funded. The study must also recommend actions that may be necessary to ensure that the system can be sustainably maintained and funded by the U.S Virgin Islands for the next 20 years.