Does the listed heavy armor bulk account for the padded armor and gauntlets, or do I need to add that on top separately?
I am assuming that only becomes relevant when someone opts to sleep in padded armour?
This is my personal take on this: Since heavy armour includes padding and gauntlets, yes, the listed bulk accounts for that.
However, since bulk is in large parts an abstraction, if someone would announce to only wear padded armour and gauntlets, and stuff the rest of their plate mail into a backpack, I would still make the packed up suit of armour count for its full encumbrance. You know, to avoid shenanigans.
If you rule that plate mail minus padding and gauntlets only takes up 3.8 bulk of backpack real estate, that probably works too though.