__gcd function in visual studio version 1.38.0

Question

I'm trying to find out the GCD in C++. I'm using __gcd code as mentioned on this website : https://www.geeksforgeeks.org/stdgcd-c-inbuilt-function-finding-gcd/

Appended is my code. Can someone please guide as to what is wrong

#include <iostream>
#include <string>
#include <numeric>
#include <algorithm>
using namespace std;

class Rational {
    private:
    int num;
    int denom;

    public:
    Rational(int a, int b){
        num = a;
        denom = b;
    }

    int add(){
        return num + denom;
    }

    int sub(){
        return num - denom;
    }

    int mul(){
        return num * denom;
    }

    void gcd(){
    cout <<__gcd(num,denom);
    }


    int simplify(){
        int gcd1 = gcd(num,denom);
        return (num/gcd1,denom/gcd1);
    }

};

int main(){
    Rational r(2,6);
    cout<<r.add()<<endl;
    cout<<r.sub()<<endl;
    cout<<r.mul()<<endl;
    cout<<r.gcd()<<endl;