The latest update of the gameboy development kit (gbdk) was a long time ago (mid 2002). Since then the SDCC compiler has gotten native support for compiling and linking fully functional gameboy roms.
However GBDK is still useful since it contains libraries for developing applications for the gameboy family. This project aims to update the libraries to be compatible with new versions of SDCC and provide helpers for building roms.
The GBDK libraries are copied from the latest source release on sourceforge: http://sourceforge.net/projects/gbdk/files/gbdk/2.96/