You add your Intelligence Modifer, or Strength if you prefer.
The Armorer’s 3rd level Arcane Armor feature states:
Each model includes a special weapon. When you attack with that weapon, you can add your Intelligence modifier, instead of Strength or Dexterity, to the attack and damage rolls.
The thunder gauntlets are the special weapon referenced here.
Alternatively, you can still choose to use your Strength. Since the gauntlets count as a simple melee weapon, the rules for damage rolls state:
When attacking with a weapon, you add your ability modifier — the same modifier used for the attack roll — to the damage.
And the rules for melee weapon attack rolls state:
The ability modifier used for a melee weapon attack is Strength