Having a problem with gcd

I'm having a problem with gcd. I don't know why it gives me a red squiggly under the gcd(lower case) at this line
GCD = gcd(y, x % y);

Any help?

Code:

// Greatest Common Divisor.cpp : Defines the entry point for the console application.
//

//	=========
//	Libraries
//	==================
   #include "stdafx.h"
   #include <iostream>
   #include <math.h>
   #include <stdio.h> 
//	==================

//  ==========
//  Namespaces
//	====================
	using namespace std;
//  ====================

//  ===================
//	Function Prototypes
//	=====================
	void WelcomeBanner();
	void GetIntegers(int&, int&);
//	=====================



// ==============

	int main( ) { // main()

		int x;
		int y;
		int GCD;

		WelcomeBanner();
		GetIntegers(x, y);

		if (y == 0 ){
			
			cout << "GCD = " << x << endl;
		}
		else
			GCD = gcd(y, x % y);
			cout << "GCD = " << GCD << endl;
		
	
		


	return 0;
	}// main()
//	========


//  =========================
//	Function WelcomeBanner()
//	=========================
	void WelcomeBanner(){

		cout << "================================================" << endl;
		cout << "Welcome to my program that will give you the   |" << endl << "greatest common divisor of your two numbers =) |" << endl;
		cout << "================================================"<< endl << endl;

	}// Welcome banner
//	==================

//	====================
//	Function GetIntegers
//	====================
	void GetIntegers(int& num1, int& num2){

		cout << "Please enter an integer that you want me to find the GCD of." << endl;
		cin >> num1;

		cout << "Please enter another integer that you want me to find the GCD of." << endl;
		cin >> num2;

	}//GetIntegers
//	==============

Bump!

Have you written the gcd function somewhere becuase there is no gcd function in cmath.

Well in my book it says

gcd(x, y) = x if y = 0
gcd(y,x%y) if y =/= 0

So I'm a little confused..

Your book gave you the mathematics that can be used to implement the function. It's your turn now

That's the concept of GCD. You have to write that gcd function yourself.

Code:

int gcd(int x, int y)
{
  translate that part into code here
}

Awwww. Okokok. I can do that