Here are some of my observations from a while back, when messing around on the online version. The premise is, if revealing/looking at cards from your deck counts as moving them off your deck...how far does that go? And also remembering from Jeebus' doc about how Armory/Blessed Village/Sun/Gatekeeper works.
Scenario 1: Armory a Fool's Gold, react with Falconer, gain and City-state a Sentinel, and topdeck FG. Gatekeeper can't exile FG.
Scenario 2: Same as scenario 1, except you City-state a Chameleon-Oracle, and topdeck the 2 cards you reveal. GK can't exile FG (even though you leave it on your deck and don't draw it).
Scenario 3: Same as scenario 1, except your deck and discard are both empty (so FG is the only card that Sentinel looks at). GK still doesn't exile FG, even though you topdecked it.
Scenario 4: Same as scenario 1, except you City-state Chameleon-Patrician instead. GK still doesn't exile FG, even though you topdecked it.
Scenario 5: Same as scenario 1, except you City-state Duchess instead. GK still doesn't exile FG, even though you topdecked it.
Scenario 6: You Falconer a FG, and react with a 2nd Falconer to City-state a Menagerie, revealing the FG. FG finally get exiled.
Obviously you can do these following scenarios with other cards; they don't all have to be Falconer and Gatekeeper. I'm only using those cards because A) Falconer is my favorite card in the game, and B) it's fun to make Gatekeeper lose track.
So I think there are 3 rulings we can go from here:
1. Revealing/looking at a card will always cards to lose track (in which case only scenario 6 is a bug).
2. Revealing/looking at a card
from your deck will always cause cards to lose track (so all those scenarios are correct).
3. Scenario 1 and 2 causes cards to lose track because you're covering up the gained card with another card from your deck; in scenario 3, 4, 5, and 6, that doesn't happen, meaning 3, 4, and 5 are bugs.
I think the 2nd ruling would be the best one. I guess the 1st ruling is ok, but it wouldn't be that intuitive. The 3rd ruling makes sense as an extrapolation, but it opens a can of worms, and why would we do that.
One issue is: "Look at" means the order you return the cards is hidden. Surely Gatekeeper can't do anything about that. But with reveal, you can't hide that order (the Hinterlands rulebook is wrong about this, but meh).
Suppose you play a Rabble, and I reveal Copper, Estate, Duchy.
Do you get to know if I put the Estate or the Duchy on top?
Yes. They were revealed, and nothing ever said to conceal them. That would also apply to e.g. Oracle.
In practice no-one would show you irl, but there would have to be a general rule to fix that, and it would be a weird general rule. The card itself sure doesn't want to say, "then conceals them, then returns them in any order they choose."
So in the Oracle example, if you put the FG 2nd on the top (which everyone knows about), it seems pretty clear that GK can't exile it. But what if you put it 1st on the top? It didn't get covered up, so can GK still find it?
Anyways, if we went with the 2nd ruling, we wouldn't need to answer these questions. And it also means Stef doesn't need to change anything. So I'm putting my vote on that.
tldr: Gatekeeper sucks at her job.