This may have sound a bit harsh, but I have no un-ease about your project at all. I'm telling you that if you want clear answers, you have to ask clear questions. Now that we have a clear question, I'll do my best to give you a clear answer.
So please can I get some advice as to where or how to include the display files with in Gamebuino.h and cpp files.
You have to replace the original Display.h and Diplay.cpp with your own. You don't have to edit any other file of the Gamebuino library.
But for it to work, your Display library
must have the same members and methods with the same arguments available. You can have extra methods too, that's not a problem.
Some methods are in the Gamebuino Display.h and not in Adafruit_GFX, for example bitmaps with rotation. You still have to implement them in your library for it to be compatible, but you can put a place-holder that only draw rectangles instead of bitmaps and actually implement the bitmap methods later on.
You also have to use the same #defines and variable names as the original Display.h. For example your error message tells that you refer to "WIDTH" while the Gamebuino #defines in Display.h are "LCDWIDTH" or "LCDWIDTH_NOROT" (depending on if you want it to be affected by screen rotation).
Finally, for the Gamebuino library there has no separate GFX class, everything have been merged into Display.h and Display.cpp.
Here it look like you include both the original Display.h, your display library LCD_2000_7775.h, and the Adafruit_GFX.h library.
1 - Restore Gamebuino.h so it only includes Display.h.
2 - Create your own Display.h and Display.cpp by merging LCD_2000_7775.h/cpp and Adafruit_GFX.h/cpp.
3 - Once it compiles correctly out of the Gamebuino library, rename the variables, methods and #defines so they match thoses from the original Display.h and Display.cpp. Then check that it still compiles.
4 - Only then, replace the original Display.h and Display.cpp with yours. The Gamebuino Library should compile properly as it can call the exact same methods with the same arguments.