Issues with Code in GCD of Two Numbers

[sheen8]

Hello everyone!

I am having difficulty understanding the code in this article on the Greatest Common Divisor of two numbers:
I am trying to implement the Euclidean Algorithm but I am getting an incorrect output. Here is my code:

def gcd(a, b): 
    if b == 0: 
        return a 
    else: 
        return gcd(b, a % b) 

print(gcd(20, 8))

The output I am getting is 8, when I am expecting 4. Can someone please explain why this is happening and how to fix it?

Thank you!