Вы можете использовать модуль random
:
import random
n = 25 #size of sample
elements = set(["GK", "A", "M", "D"])
random_pop = [element for i in xrange(n) for element in random.sample(elements, 1)]
>>>random_pop #example run
['M', 'D', 'M', 'M', 'D', 'A', 'M', 'A', 'M', 'GK', 'GK', 'D', 'GK', 'A', 'M', 'D', 'A', 'A', 'GK', 'M', 'D', 'M', 'A', 'GK', 'GK']