Can the US Government release a program under the GNU GPL?
If the program is written by US federal government employees in the course of their employment, it is in the public domain, which means it is not copyrighted. Since the GNU GPL is based on copyright, such a program cannot be released under the GNU GPL. (It can still be free software, however; a public domain program is free.) However, when a US federal government agency uses contractors to develop software, that is a different situation. The contract can require the contractor to release it under the GNU GPL. (GNU Ada was developed in this way.) Or the contract can assign the copyright to the government agency, which can then release the software under the GNU GPL.
If the program is written by US government employees, it is in the public domain, which means it is not copyrighted. Since the GNU GPL is based on copyright, such a program cannot be released under the GNU GPL. (It can still be free software, however; a public domain program is free.) However, when a US government agency uses contractors to develop software, that is a different situation. The contract can require the contractor to release it under the GNU GPL. (GNU Ada was developed in this way.) Or the contract can assign the copyright to the government agency, which can then release the software under the GNU GPL.