Create a function that takes a list of numbers and returns the greatest common factor of those numbers.
gcd([84, 70, 42, 56]) ➞ 14
gcd([19, 38, 76, 133]) ➞ 19
gcd([120, 300, 95, 425, 625]) ➞ 5
The GCD is the largest factor that divides all numbers in the list.