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.
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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.