They probably shouldn't "stack"
For the record, the text of the two abilities in question is reproduced here.
Guardian Armor comes with Thunder Gauntlets:
Thunder Gauntlets. Each of the armor's gauntlets counts as a simple melee weapon while you aren't holding anything in it, and it deals ld8 thunder damage on a hit. A creature hit by the gauntlet has disadvantage on attack rolls against targets other than you until the start of your next turn...
The Arcane Propulsion Armor infusion says, among other benefits:
- The armor includes gauntlets, each of which is a magic melee weapon that can be wielded only when the hand is holding nothing. The wearer is proficient with the gauntlets, and each one deals ld8 force damage on a hit and has the thrown property, with a normal range of 20 feet and a long range of 60 feet. When thrown, the gauntlet
detaches and flies at the attack's target, then immediately returns to the wearer and reattaches.
Now you can clearly have both effects active at the same time there's no reason you can't infuse your chestplate with the Arcane Propulsion Armor infusion and thereby gain all the benefits of the infusion.
However, while it isn't explicit, I think "stacking" should be disallowed. I would agree with the people who compare this to an AC calculation; my reading of this is that the gauntlets have two 'modes' and you can pick which one you want to use for any given attack. In one mode, you deal thunder damage and encourage the enemy to target you; in the other you deal force damage and can make ranged attacks.
Allowing both effects at once would potentially mean your attacks deal 2d8+Int damage on a hit, which would perhaps not be entirely broken, but it seems contrary to the intent of either item.
If your intent here is to merely get your gauntlets to make 1d8-damage ranged attacks that inflict disadvantage if the target doesn't attack you, that's less broken, and while I understand what you're looking for, it doesn't seem like that should work. If nothing else, it becomes ambiguous what damage type your gauntlets actually deal in that case.
Your DM may be all right with you making rocket-punch attacks that cause the disadvantage effect, but I wouldn't allow that in my game. The simple answer is you have two weapons at your disposal, like any weapon-using character who carries a sword and a throwing axe.