Mechanically, gauntlets do the same thing as other parts of the armor set listed in the item description - they protect the wearer, as a set. Like the helmet of plate armor, gauntlets are called out particularly, because wearing them might preclude wearing other items.
There are no “item slots” — use the item description instead
Fifth edition does not use a concept of item “slots” that fit on parts of the body. Instead, the item description details how a given type of armor is worn. Not surprisingly, armor that offers more protection requires wearing armor over more of the body. This can preclude wearing other useful items.
Wearing gauntlets means you cannot be wearing gloves
When a suit of armor specifies it includes gauntlets, you are not exactly wearing that type of armor when you are not wearing gauntlets. Since, “a character can’t normally wear more than one pair…of gloves or gauntlets,” (DMG p. 141) that means that if you are wearing magic gloves instead of gauntlets, you are not wearing your full set of armor. What this means mechanically is not spelled out, and would be left to DM discretion. (A -1 AC penalty would be plenty).
The item description defines what a set of magic armor includes
For mundane armor, the gauntlets included with the could be swapped out for another pair, such as Gauntlets of Ogre Power.
On the other hand, for magic armor that is a type that includes gauntlets, the gauntlets are part of the magic item. For example, plate armor comes in several pieces; no one piece is the magic item. What composes the armor “item” is defined in item description (in the PH, p. 144-145).
You can’t turn Platemail of Etherealness into Breastplate of Etherealness by only donning some of the armor. Likewise, you can’t swap out the gauntlets (or helmet) from the set, and still get the etherealness. Just as you need to wear both magic boots or gloves in a pair (see Paired Items, PH p. 141) you need to wear all pieces of the magic armor, including the gauntlets, to gain its benefits.