Ultimately this is truth. Had those gauntlets been adamantine and not magical, but appeared identical to how the DM described, the players would have sold it as part of the set and nobody here would say boo about it.
Of course. Because if the gauntlets had had no special significance, and hence had not been something that the players had distinctive concerns for, then by "the armour" the player would have intended to refer to the armour and the gauntlets.
No one is denying that reference is driven, in part, by what speakers care about and what is salient to them. On the contrary: that's the basis for criticising the OP's adjudication of the ranger player's action declaration!
Any aspect of play that players may care about is worth paying attention to. As a player, I would care quite a bit about unidentified magic items. Such items could be meaningful to the adventure beyond their use as simple gear. Until such items are identified they wouldn't be tossed into a bundle and treated as mere inventory.
Telling me what you do as a player isn't answering the question, though.
I, as a player, am not very interested in inventory management, in detailing how items are bundled up, etc. It's not a part of the game that really grips me. Is that a flaw, as a player? Is it bad play to assume that the GM will recognise that saying "I sell the armour" doesn't also mean "And I give away our magical gauntlets for free", in circumstances where I know there are magical gauntlets around but haven't bothered to note exactly how another player declared them to have been bundled up?
I don't think that it is.
When something is clearly part of a set like that, you need to let the DM know if you are doing something outside the normal
In what way were the gauntlets clearly part of a set? The GM knew they were distinctively magical. The players knew they were distinctively magical. The GM knew that the players didn't mean to sell them (or, in fact, give them away) - hence the "dramatic" revelation that this was what had happened.
There is no uncertainty here. No one's mental state was one of confusion. The issue is purely about the
wording of an action declaration.
The player hadn't
forgotten about the gauntlets. The player didn't want to sell the gauntlets. The player had told the GM that his PC was selling something that they both knew to be distinct from the gauntlets - namely, the non-magical armour.
It's not about mental states. It's about wording.