User LiQuid.Ace on GPForums found out that every 5 digit number that is a multiple of 3 gives a FREE garlic bread classic cheese pizza with any pizza. I think that each code only works once but as there are so many codes that shouldn't be a problem. I've tested a few random codes and they all seem to work.
If you can't think of a 5 digit number:
1. Go to Random.org.
2. Generate a random in between 1 and 33333.
3. Multiply your number by 3.
Doesn't work for the $5 ones at least. You'll see it in the cart but the order confirmation will not have it. I double checked with the lady at Pizza Hut as well but she said that their copy didn't say anything about a free garlic bread.