What is the Government Pension Offset?
If you worked in a job in which you did not pay Social Security taxes (such as the Civil Service Retirement System, although not CSRS Offset or FERS), some or all of a Social Security spousal or survivor benefit for which you may be qualified may be offset under the Government Pension Offset, sometimes called the Public Pension Offset.

The offset will reduce the amount of your Social Security spousal or survivor benefits by two-thirds of the amount of your non-Social Security annuity. In other words, if you get a monthly CSRS annuity of $1,200, two-thirds of that, or $800, must be used to offset your Social Security spousal or survivor benefits. In some cases it will eliminate the spousal or survivor benefit entirely, in the same way that couples who both earn Social Security benefits will receive only the higher of their own earned benefit or the spousal or survivor benefit.

There are some exceptions. See Chapter 3, Section 9 of the Federal Employees Almanac.