dnd 5e – Can an Armorer artificer using the Guardian armor model wield a shield or weapon in one Thunder Gauntlet, and still attack with the other gauntlet?

Each gauntlet is an individual melee weapon.

The Thunder Gauntlets state:

Each of the armor’s gauntlets counts as a simple melee weapon while you aren’t holding anything in it.

This indicates that each gauntlet is individually a simple melee weapon, each wielded in one hand.

So you can wield something in one hand and use a Thunder Gauntlet with the other, with all the benefits a Thunder Gauntlet brings.