If I’m wearing a set of full plate armor and I replace the gauntlets with gauntlets of ogre strength do I lose the AC provided by the full plate as I’m no longer wearing a full set of full plate armor?
No, because the rules don't say so
The rules don't really manage that level of granularity with armor. Plate is plate, and if you take off the boots or gloves, the rules don't seem to care. (thanks @ThomasMarkov for the summarizing)
Gauntlets of Ogre Power description doesn't say the item decreases a full plate's AC, so it doesn't. There are no secret rules:
Beware of claims that a rule does something mentioned nowhere in that rule or elsewhere in the core books. There aren't secret rules. (source)
From the simulationist's point of view, one can rationalize it as the character replaces one pair of gauntlets with another one, so the "total" AC remains the same.
- Is partial armour an optional rule?
- Combining various AC-providing clothing and armor
- Do gauntlets do anything?
The Standard Caveat™: D&D 5th edition enforces the "rulings over rules" principle — it empowers the DM in ways that 3rd, 3.5, and 4th did not. While rule zero has always applied, 5th edition chooses not to explicitly codify many things. If your DM says your AC is lowered, it is. However, I've never seen anyone justified wearing magic gauntlets this way.