From Wikipedia: The Game System License is a license that allows third-party publishers to create products compatible with and using the intellectual property from Dungeons & Dragons (4th Edition). It was released by Wizards of the Coast to the public on June 17, 2008. ... The GSL grants use of the 4th Edition System Reference Document, which lists trademarks, words, and short phrases that could be used to refer to materials in the Dungeons & Dragons 4th edition rules, but contains no rules itself. People wishing to use this license are also granted a logo that must be placed onto their products to state that they are compatible with Dungeon & Dragons 4th Edition. The license also can be updated by Wizards of the Coast and updates affect all licensees; in case of litigation the licensees must pay the legal costs of Wizards of the Coast.