If you take a look inside DarkGDK.h you see these lines :
// Product: DarkGDK (Dark Game Development Kit)
#define DarkSDK DarkGDK
#define LoopSDK LoopGDK
#include "DarkSDK.h"
So DarkGDK.h seems to include Darksdk.h (Which seemed to be the old sdk). Inside darksdk you can see that every "chapter" of the manual is included. DarkSDKCore.h corresponds to the core functions, darkSDKBasic3d corresponds to the basic 3d functions and so on. These are mostly replicated from the commands in Dark basic Pro.
These header files in turn contains the function declarations (but not the definitions since these are header files).