Multiple Source .cpp Files Problems

The problem is that you've included .cpp files, that's why you had to tell Visual Studio to pretend that these files were not "source" code, because it always compiles source code files. So the simple solution is don't do that.

Including just the header files (.h) in each .cpp file as required should be sufficient. So, for example, if gui.cpp needs to call functions defined in display.cpp, then you should #include the display.h header file at the top of the gui.cpp code file.

Use forward declarations to eliminate any lingering circular dependencies.

That is... not the right way to approach things. If I had to speculate, I'd guess you aren't using function declarations that aren't definitions. A function definition is what you probably have:

void dothing(int param) { //function definition
    throw param;  
}

What goes in the header is the function declaration, which would be like this:

void dothing(int param); //function declaration

This merely lets other cpp files know that the file exists for calling, but they don't need to know the details. Generally, functions will go in your .cpp files as you (seem to have) done. Each function will have a declaration in a header file:

display.h

#include "velocity.h" //we need the details, so we include

void dothing(int param); //function declaration

class  coordinate; // this is a class declaration
                   //this header needs to know about coordinates
                   //but doesn't need the details
                   //otherwise we'd have to include "coordinate.h"

struct object { //class definitions usually go in headers
    coordinate* member; //pointers don't need to know the details
    velocity speed; //actual object needs the details
    float function(float param); //function declaration
};

display.cpp

void dothing(int param) { //function definition
    throw param;  
}

float object::function(float param) { //member function definition
}

Then, if a cpp file neads access to a function in another cpp file, it includes the appropriate header:

main.cpp

#include "display.h"

int main() {
    object a;
    a.function(3.4); //can call the member just fine
    dothing(4); //it can call the function just fine.
}

Remember that headers should prefer declaring a class (in my example: coordinate to including more headers wherever possible. Also remember that templates have completely different rules altogether.

I had a similar problem before in which I had two header files referencing each other, calling functions, etc. To fix it, I first made sure that all of my header guards were in check (#ifndef HEADER_DEFINE, #pragma once, etc.), then in the class files I would include the header that was being problematic.

e.g. if I have Pie.h and Cherries.h, and Pie.h included Cherries.h and vice versa, then in the Pie.cpp file, I had the include for Cherries.h (and vice versa).