Well, trivially:

    $ patch --version
    GNU patch 2.7.6
    Copyright (C) 2003, 2009-2012 Free Software Foundation, Inc.
    Copyright (C) 1988 Larry Wall

    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
    This is free software: you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.

    Written by Larry Wall and Paul Eggert

But also, the GPL requires keeping a detailed ChangeLog, and if anyone's going to be a stickler about that, it's the FSF: https://git.savannah.gnu.org/cgit/patch.git/tree/ChangeLog-2...