Simple modification of Hello World - c++

Question

I'm a beginner C++ programmer. There has to be simple mistake somewhere but I can't figure it out why it won't compile.
Main.cpp:
#include <iostream>
#include <string>
#include "GUI.h"
#include "GUI.cpp"
int main()
{
Display("Hello World!");
return 0;
}
GUI.h
#pragma once
void Display(std::string param0);
GUI.cpp
#include "GUI.h"
void Display(std::string param0)
{
std::cout << param0;
}
Errors are:
Namespace "std" has no member "string" and "cout"
'String' is not a member of 'std'
'String' undeclared identifier
syntax error: missing ')' before identifier 'param0'
'{': missing function header (old-style formal list?)
However, when I paste code directly into Main.cpp creating
#include <iostream>
#include <string>
void Display(std::string param0);
void Display(std::string param0)
{
std::cout << param0;
}
int main()
{
Display("Hello World!");
return 0;
}
It works fine so the problem lies probably with incorrect use of #includes.
I always thought that the include directive just "injects" code into main.cpp and it's just for organizational purposes and for dividing code into smaller pieces but now I'm confused.
How this code should look like and why?
Are there already any bad programming habits in this piece of code?
EDIT: Thanks for help, it finally compiles so I assume it should be done that way:
Main.cpp
#include <iostream>
#include <string>
#include "GUI.h"
int main()
{
Display("Hello World!");
return 0;
}
GUI.h
#pragma once
#include <iostream>
#include <string>
void Display(std::string param0);
can also be done as
#ifndef GUI_H
#define GUI_H
#include <iostream>
#include <string>
void Display(std::string param0);
#endif
GUI.cpp
#include "GUI.h"
void Display(std::string param0)
{
std::cout << param0;
}

Answer 1

You have forgot to add #include <string> into GUI.h. There is no need to include GUI.cpp file.