input: Write a python function to calculate the greatest common divisor (GCD) of a list of integers.
  assert gcd([30, 42]) == 6
  assert gcd([12, 15, 21]) == 3
  assert gcd([81, 27, 45]) == 9
  assert gcd([10, 25, 50]) == 5
output: def gcd(numbers):
      from math import gcd
      from functools import reduce
      return reduce(gcd, numbers)