GYP is a Meta-Build system: a build system that generates other build systems.
Source code: https://chromium.googlesource.com/external/gyp
Mailing list: http://groups.google.com/group/gyp-developer
GYP is intended to support large projects that need to be built on multiple platforms (e.g., Mac, Windows, Linux), and where it is important that the project can be built using the IDEs that are popular on each platform as if the project is a “native” one.
It can be used to generate XCode projects, Visual Studio projects, Ninja build files, and Makefiles. In each case GYP's goal is to replicate as closely as possible the way one would set up a native build of the project using the IDE.
GYP can also be used to generate “hybrid” projects that provide the IDE scaffolding for a nice user experience but call out to Ninja to do the actual building (which is usually much faster than the native build systems of the IDEs).
For more information on GYP, click on the links above.