The weapon damage dice that have their damage type changed can be rerolled
The intent of the Great Weapon Fighting Style is spelled out in the Sage Advice Compendium:
Q. If you use Great Weapon Fighting with a feature like Divine Smite or a spell like hex, do you get to reroll any 1 or 2 you roll for the extra damage?
A. The Great Weapon Fighting feature—which is shared by fighters and paladins—is meant to benefit only the damage roll of the weapon used with the feature. For example, if you use a greatsword with the feature, you can reroll any 1 or 2 you roll on the weapon’s 2d6. If you’re a paladin and use Divine Smite with the greatsword, Great Weapon Fighting doesn’t let you reroll a 1 or 2 that you roll for the damage of Divine Smite.
It only impacts the damage roll of the weapon, so only the weapon damage dice themselves. Grave Touched does not change that the damage dice come from the weapon itself; it exclusively changes the damage type. So any weapon damage dice can still be rerolled with Great Weapon Fighting.
The intent is that the additional damage die cannot be rerolled
When a feature adds weapon damage dice it states so. Let's compare the Cavalier Fighter's Unwavering Mask feature, the Half-Orc's Savage Attacks trait, and the Barbarian's Brutal Critical feature:
[...] the attack's weapon deals extra damage to the target equal to half your fighter level. [...]
[...] you can roll one of the weapon’s damage dice one additional time [...]
[...] you can roll one additional weapon damage die [...]
All of these features specifically mention that the weapon itself gains additional damage or that you specifically roll additional weapon damage dice. Weapon damage dice are precisely what Great Weapon Fighting allows you to reroll but the Grave Touched feature does not add weapon damage dice. Instead it states:
[...] you can roll one additional damage die [...]
Because this additional die is not a weapon damage die, it cannot be rerolled with Great Weapon Fighting.
The rules as they are written, do let the additional damage die be rerolled
That entire last section relies on trusting the Sage Advice Compendium and that SAC answer even states that Great Weapon Fighting is only meant to benefit the damage roll of the weapon. If we look at the actual wording of the feature, the text does not support this stance:
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 [...]
This simply activates when you roll a damage die, not a weapon damage die. So following the rules as they are written, we actually could reroll Divine Smite, hex, Sneak Attack, or even Grave Touched. However, we know that this is not the intent of the designers. Do with this knowledge what you will.