GBDK-2020 is a development kit and a library that allows you to create SMS/GG homebrew using C. It is self-contained: bundled with the SDCC compiler, assembler, rom making tools, etc. No other tools are needed, except an emulator. It also features cross-platform examples.
Most recent build of GBDK-2020 with cross-platform support
Online documentation (no SMS/GG docs yet, work in progress)