You can just add,
while computer1 in GuessList: computer1 = randint(0, 5)
After your computer1 = randint(0, 5). You might have to remove your GuessList = [ ] because that will reset your GuessList each time. Instead, you should have that at the very start of your code and leave the append statement.
Or you can use
computer1 = random.choice(GuessesLeft)
GuessesLeft is a list of all the numbers computer1 has not guessed. Each time it makes a guess youc an just delete that item from the list.