The GDK (Gamemode Development Kit) is a library that allows you to write SA-MP gamemodes in C/C++. It is built on top of the SA-MP plugin SDK, so gamemodes written with it are basically just ordinary plugins.

The library provides most the functions you would find in Pawn, except thos that overlap with the C standard library, like file I/O or string utiluties, or could be easily implemented on top of it. It also lets you handle various SA-MP events (known as "callbacks" in Pawn).

Visit the official SA-MP forums topic for more information and support.

You can begin with downloading the source code and playing a bit with the HelloWorld plugin in plugins/helloworld.

If you know Git the simplest way to get started is probably to clone this repo and create a new branch for your personal project like my-project, and then just start developing your own plugin right in the GDK's source tree. Later if you decide to update the library you would simply switch to master, pull in the changes and merge them into my-project.

There are also some useful files provied for developing plugins in Visual Studio using the above method. They are located in build/visual-studio. Running the configure.bat script is the first thing you would do after downloading the source code and installing the necessary development tools (like CMake and the Visual Studio itself).

Next you have to set the SAMP_SERVER_ROOT environment variable to the full path of the server folder - this makes the post-build events work and allows you to run the server easily from within Visual Studio. The post-build events defined in the .user files copy both the library and helloworld.dll to %SAMP_SERVER_ROOT% and %SAMP_SERVER_ROOT%/plugins respectively (but only in Debug builds).

Pre-compiled binaries for Windows and Linux along with the documentation and other development files can be downloaded here.