Games have always been one of the most technically complex forms of media created. They are limited only by creators' imagination and their ability to translate that vision to reality through technology. We want to enable any creator, anywhere on the planet, to make games for our ecosystem. And, to help with that we've made the Microsoft Game Development Kit (GDK) available on GitHub, free of charge!

The GDK is the same base development kit used by hundreds of game creators today and was previously only available to approved partners building for the Xbox ecosystem. The GDK contains the common tools, libraries, and documentation needed for developers, it's the future of the Xbox ecosystem across all platforms—PC, cloud, mobile, and console.
Game Stack - Meet the Microsoft Game Developer Kit (GDK) | Microsoft Developer