In Adventurers League, can you wear magical boots/gloves/gauntlets while wearing plate armor?
Yes, you can add anything but another armor
While not as explicitly as in previous editions, magic items occupy distinct slots. A boot occupies your feet slot, a hat occupies your head slot, and so on.
This is indirectly mentioned in the Multiple Items of the Same Kind section of the rules:
[...] A character can’t normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or be able to layer two cloaks.
Your armor, regardless of its appearance, occupies your armor slot, but does not prevent wearing unrelated footwear or even bracers.
Historically, plate armor came with helmet and boots, but D&D is not a simulationist game.
Consequently, you can match any armor with any helmet, glove or boot, but you cannot just wear the breastplate part of your half plate when you want to avoid disadvantage on stealth.
The Player's Handbook is clear that:
Plate consists of shaped, interlocking metal plates to cover the entire body. A suit of plate includes gauntlets, heavy leather boots, a visored helmet, and thick layers of padding underneath the armor. Buckles and straps distribute the weight over the body.
The armor you are wearing already includes gauntlets, boots, and a helmet. (Another question on this site tackles the force-of-rule that these descriptions have on available armor in the Adventurer's League)
The Dungeon Master's Guide describes what to do in this situation:
Use common sense to determine whether more than one of a given kind of magic item can be worn. A character can’t normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or be able to layer two cloaks.
However, this rule only applies to wearing "more than one of a given kind of magic item" and says nothing about wearing a magical gauntlet, with the mundane gauntlets of plate armor. With regard to that scenario, the rules are silent. You will have to ask a GM how they will be interpreting these rules to cover the ability to wear armor pieces of both mundane and magic quality.
Note: you should be able to wear gloves under your gauntlets, or a circlet under your helmet, or slippers under your boots with GM permission