Grand Unified Builder (GUB) is a mini source-based distribution and packaging system. It cross-compiles several packages with their dependencies and assembles them into a single installation package. It currently supports GNU/Linux, FreeBSD, Mac OS X, and Microsoft Windows installation packages. All packages are fully relocatable and can be installed with user privileges. Plain Python classes are used to describe package build specifics. For reasonably well behaved packages, individual build specifications can stay very simple by extending the generic build specification classes. It was originally developed for turnkey building of all LilyPond binary installers.