What is the time complexity of __gcd?

What is the time complexity of __gcd(m,n) function? Also, does it use the Euclidean method to calculate gcd?

e.g. code

#include <iostream> 
#include <algorithm> 

using namespace std; 

int main() { 
    cout << "gcd(10, 25) : " << __gcd(6, 20) << endl; 
}