Z:gnu-www-ja-gpl-2.0-faq--15b9ac-If the program is written by U/en

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.)