input: Write a python function to find the greatest common divisor (GCD) of two numbers.
  assert get_gcd(7, 8) == 1
  assert get_gcd(12, 15) == 3
  assert get_gcd(100, 10) == 10
output: def get_gcd(a, b):
      while b:
          a, b = b, a % b
      return a