“Gods & Monsters” is a fantasy role-playing game released under the Gnu Free Documentation License. More information about the license is available within the game book and at the Gnu FDL web site.

Open gaming basically means is that while “Gods & Monsters” is copyrighted, the license allows you to use the game, edit it, and modify it, as long as you agree that your modifications are also available under the Gnu Free Documentation License.

Of course, you can make non-FDL, “Gods & Monsters”-compatible adventures and add-ons without infringing on the copyright; just don’t copy or edit the text from “Gods & Monsters” to make your “aftermarket accessory” or otherwise transform the work. It is only when a copyright is infringed or a derivative work is created that a license such as the FDL is invoked.