Hi Muffin Man.
If you are going to make a First Person Shoot 'em up, and you want to release it under the GPL, then you should really read the link I will give you. Now you will not be interested in the first few options (unless you have a cool US$250,000.00) but they have a basic GPL version for free, there are quite explicit terms and conditions for some options, but it might suit your purposes. I would strongly suggest you go through it and see if it might help. If nothing else it would make useful reading.
http://www.idsoftware.com/business/home/technology/Hope this is of some help.
[ April 11, 2003: Message edited by: Linux Frank ]