
Here's the simple steps to attach a GPL license to your source files (written after requests by DiggClone [1] and Bandnet [2]):
For your own project, just add the following text-notice to the header/first section of each of your source-files, commented out in whatever way your language uses:
----------------following is the notice-----------------
 /*
 * Your Project Name -  -you slogan-
 * Copyright (C) 2007 - 2007 Your Name
 *
 * This program 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.
 *
 * This program 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 this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */
 ----------------------------------------------
the "2007 - 2007" needs to be adjusted  to "year when you gave it the license in the first place" - "current year". 
Then put the file gpl.txt into the source-folder or a docs folder: http://www.gnu.org/licenses/gpl.txt [3]
If you are developing together with other people, you need their permission to put the project under the GPL.
------
Just for additional Info, I found this license comparision paper by sun: http://mediacast.sun.com/share/webmink/SunLicensingWhitePaper042006.pdf [4]
And comments to it: http://blogs.sun.com/webmink/entry/open_source_licensing_paper#comments [5]
It does look nice, but it misses one point:
GPL is trust: Contributors can trust, that their contributions will keep helping the community, and that the software they contribute to will keep being accessible for the community.
(That's why I decided some years ago to only support GPL projects. My contributions to one semi-closed project got lost, because the project wasn't free and the developer just decided not to offer them anymore, and I could only watch hundreds of hours of work disappear, and that hurt.)
Best wishes,
Arne
PS: If anything's missing, please write a comment!
Links:
[1] http://www.talkingpixels.org/diggclone/index.php
[2] http://www.bandnet.org
[3] http://www.gnu.org/licenses/gpl.txt
[4] http://mediacast.sun.com/share/webmink/SunLicensingWhitePaper042006.pdf
[5] http://blogs.sun.com/webmink/entry/open_source_licensing_paper#comments