licencing is one thing - copyright is another.
if you make some programm (or game) open source under the gpl - your code is freely available to everybody - but has to refer back to you when using snippits or the entire code - and people are free to make changes and redestribute.
the story though does not have to be released under the same licence, as your gamecode and your story can also be physically seperate, and you can choos a license which would protect your copyright.
copyright is dependant on the country you live in. but a lot eaier to get - (again depending on the country) in most european countries (e.g.) you have a copyright on anything you make, any photo you have taken, etc.
but you need means of proof. an easy way of proof (again i only know about european countries) is to burn the data on a cd, slip it in an evelope and post it to yourself. once you get your letter, do not open it, but keep it safe.
the date on the stamp will count as your copyright date for the contents of the cd and is recognized as valid by every solicitor.