gcd<int64_t>(x,y)

__gcd <long long> (x, y)

Is __gcd wrong with two long long variables? I always use it and have not seen any problems. A test case would be helpful. Thank you

Why rely on an obscure, non-standard function (that might not work everywhere) when you can just implement the Euclidean algorithm yourself with only three lines of code (not counting braces)? If even that amount of typing bothers you, you can add it to your template.

long long GCD(long long x, long long y)
{
    if (y == 0) return x;
    return GCD(y, x%y);
}