Why am I getting this redefinition of class error?

You're defining the class in the header file, include the header file into a *.cpp file and define the class a second time because the first definition is dragged into the translation unit by the header file. But only one gameObject class definition is allowed per translation unit.

You actually don't need to define the class a second time just to implement the functions. Implement the functions like this:

#include "gameObject.h"

gameObject::gameObject(int inx, int iny)
{
    x = inx;
    y = iny;
}

int gameObject::add()
{
    return x+y;
}

etc

add in header files

#pragma once