The game code is the central part of the source code shipped with the SDK. By loose definition, game code includes all source code in src/game
Default SDK Installation Directory: C:\Quake4_SDK Game Code: C:\Quake4_SDK\src\game
The game code is compiled and linked into a shared library that is then loaded by the engine. The shared library differs by platform:
Windows: gamex86.dll Linux/BSD: gamex86.so
Game Code Overviews
GameCodeByFile - Overview of the game code by file