The Government Pension Offset (GPO) is a social security rule that affects workers with government pensions who also receive social security benefits. It reduces spousal or survivor benefits by an amount equal to two-thirds the amount of their public pension. Spousal and survivor benefits are generally available to anyone whose deceased or retired spouse worked at a job in which they earned Social Security Benefits, and the GPO can greatly reduce those benefits.
The GPO affects people who worked as federal, state, or local government employees if the job was not covered by Social Security. Most common professions affected by this are teachers, law enforcement, postal workers, firefighters, and other government employees. These workers are employed by an employer that has its own pension system and does not participate in social security such as CALSTRS for teachers in California. You can tell by looking at the Social security statement and see zeros in the Taxed Social Security Earnings with income reported in the Taxed Medical Earnings.
It is great if you are covered by a government pension. We just want to make sure you are not overestimating what you have as income in retirement by counting a Social Security benefit that may be reduced.