a recursive method will work perfectly

int gcd(int a, int b) { if (a == 0) return b; else return gcd (b % a, a); }

Link

Euclidean algorithm

Euclidean Algorithm is the most efficient for calculating GCD.

Method 1(Recursive) :

int gcd_number(int a, int b) {
    if (b==0) {
        return a;
    }
    else {
        return gcd_number(b, a-b*floor(a/b));
    }
}

Method 2 (Recursive) :

int gcd_number(int a, int b) {
    if (a==0) {
        return b;
    }
    return gcd_number(b%a, a);
}