Yes it is on github, in scripts folder alongisde previous build scripts. It is made of several makefiles: Makefile.def, Makefile.gid, and Gid*.mk.
To use it, first customize scripts/Makefile.def.example according to your computer configuration, then launch the build from a command prompt in gideros root folder by issuing 'make -f scripts/Makefile.gid '.
Target can be empty for building everything that can be built on that platform, or be a specific target to be built, for ex: qtapp.install to build studio, html5.install to build Html5 template, android.install, to build android stuff and so on. Have a look at the content of the Makefiles to find other targets.
BUT: you need a windows machine to build windows stuff (and WinRT), and a Mac to build Mac and iOS stuff, and the makefiles heavily assume that you build mainly on windows (because thats my case)