D&D 5e does not distinguish between main and off hand.
Nowhere do the rules make a distinction between main hand and off hand.
The Thunder Gauntlets are not light melee weapons.
Now, for the purposes of two weapon fighting, you cannot use the gauntlets to make a bonus action attack, as two weapon fighting requires each weapon to be a light melee weapon:
When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand. You don’t add your ability modifier to the damage of the bonus attack, unless that modifier is negative.
The gauntlets do not have the light property:
Each of the armor’s gauntlets counts as a simple melee weapon while you aren’t holding anything in it.
They both get the bonus.
The “special weapon” of the Guardian Armorer is:
Each of the armor’s gauntlets
Each gauntlet is itself a weapon, and each gets the bonus from your intelligence modifier to attack and damage rolls.
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.
I know it says “weapon”, singular, and the guardian technically gets two, one for each hand.