GWF let's you reroll the die if it's a 1 and 2, so in the instance of a Greatsword do you reroll only one of those die if both land on a 1 or a 2, or both? I imagine if you could reroll both it would say dice but maybe that's just me misreading it.
Yes, you can reroll each of your attack dice that rolled a 1 or a 2.
Just for reference, this is what it says in the PHB:
When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2.
The "When" here is the same as "Whenever". Also, the rule applies to "a" die, not "the" die. The correct way to interpret this is that you apply the rule to each die individually, no matter how many you roll.
If you're worried about the balance of the feat when it comes to multi-die weapons, AceCalhoon has all the numbers in this great answer to a related question.