I'm getting a "name not defined error" when I run my program, even though I have defined it in the class.

Here is my Python code:

```
class Fraction(object):
    """docstring for Fraction"""
    def __init__(self, numerator, denominator):
        self.numerator = numerator
        self.denominator = denominator

    def show(self):
        print(self.numerator, "/", self.denominator)

    def __str__(self):
        return str(self.numerator) + "/" + str(self.denominator)

    def __add__(self, other_fraction):
        new_numerator = self.numerator*other_fraction.denominator + self.denominator*other_fraction.numerator
        new_denominator = self.denominator*other_fraction.denominator

        common = gcd(new_numerator, new_denominator)
        return Fraction(new_numerator//common, new_denominator//common)

    def gcd(numerator, denominator):
        if numerator % denominator == 0:
            return denominator
        else:
            return gcd(denominator, numerator % denominator)

fraction1 = Fraction(3, 5)
fraction2 = Fraction(8, 9)

print(fraction1 + fraction2)

```

So you guys can tell me some wrong in above code when I execute and catch an error as bellow:

```
Traceback (most recent call last):
  File "fraction.py", line 31, in <module>
    print(fraction1 + fraction2)
  File "fraction.py", line 17, in __add__
    common = Fraction.gcd(new_numerator, new_denominator)
  File "fraction.py", line 24, in gcd
    return gcd(denominator, numerator % denominator)
NameError: name 'gcd' is not defined

```