function returns "none" if called in another function

Question

I want to get the euclidean totient, but when i call the ggt(x,z1) in totient() it returns None. Any idea what the problem is? Shouldn't ggt return the value to the totitent()?

#!/usr/bin/python3

def ggt(a, b):
    if a > b:
        while b!=0:
            c = a % b
            a = b
            b = c
        return a

    else:
        ggt(b,a)

def totient():
    while True:
        try:
            z1 = int(input('insert number '))
            break
        except:
            print("Not a integer!")
            continue

    counter = 1

    for x in range(2,z1):
        cache = ggt(x,z1)
        if (cache == 1): # <-- this returns None
            counter += 1

    print("phi(%d)" % (counter))

totient()