input: Write a function to calculate the greatest common divisor (GCD) of two numbers and return the result. The function should use the Euclidean algorithm to determine the GCD. If the second number is zero, the first number is the GCD.
  assert gcd(10, 20) == 10
  assert gcd(25, 15) == 5
  assert gcd(17, 13) == 1
  assert gcd(48, 18) == 6
  assert gcd(0, 10) == 10
output: def gcd(a, b):
      while b != 0:
          a, b = b, a % b
      return a