This plugin is an extension to the per-player variable (PVar) system introduced in SA-MP 0.3a with a few key differences:
Open the solution file (gvar.sln) in Microsoft Visual Studio 2010 or higher. Build the project.
Install the GNU Compiler Collection and GNU Make. Type "make" in the top directory to compile the source code.
The latest binaries for Windows and Linux can be found here.
More information can be found in the SA-MP forum thread as well as the README file in the binary package.