code: public static BigInteger gcd(BigInteger value1, BigInteger value2) {
      return INSTANCE.get().gcdImpl(value1, value2);
    }
docs: Return the greatest common divisor of value1 and value2. The result is always positive even if
  one or both input operands are negative. Except if both inputs are zero; then this method
  defines gcd(0,0) = 0.
  
  @return greatest common divisor of value1 and value2