What is Government Accountability Office?

Legal Definition
The Government Accountability Office (GAO) is a government agency that provides auditing, evaluation, and investigative services for the United States Congress. It is the supreme audit institution of the federal government of the United States.
-- Wikipedia
Legal Definition
The Government Accountability Office (GAO, previously called the Government Accounting Office) is the legislative agency responsible for ensuring the funds appropriated by Congress are spent for the purposes provided by Congress.

GAO engages in audits and investigations, but it has negligible enforcement power. Once a legal determination has been made, GAO has exhausted its regulatory authority. Regardless of the adjudicative outcome, GAO has no authority to exact fines, issue injunctions, or pursue further proceedings, criminal or otherwise. Instead, the Comptroller General reports the determination to Congress, to the president, to the offending agency, and to any other relevant agencies (such as the Department of Justice).