Basically, all the Open Arena stuff gets put into one (compressed/zipped) file, which includes a COPYING file (with the text of the GNU GPL) and an AUTHORS file, with, uhm, the authors' names. The file you put for download here isn't licensed under the GNU GPL though, because it has no COPYING file with the GNU GPL inside, Also, you should include a note in every file that it's licensed under the GNU GPL:
This file is part of Open Arena.
Open Arena is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
Open Arena is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with Foobar; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
[Edited on 2005-10-18 by Hieronymus]