I today received the below ruling through the official rules question forum, which gives this weakness rather more teeth than I and others originally anticipated:

"If Gloria looks at multiple copies of Prophecy of the End with her ability, each “Forced” ability on the treacheries resolves, and each are placed facedown beneath Gloria."

As written this weakness sounds scary (not many weaknesses have the power to instantly defeat an investigator after all), but the insta-kill clause should never fire if you know what you're doing.

Note that the Forced effect only triggers if Gloria actually chooses to use her when looking at the encounter deck. As such, I believe that it is entirely possible for Gloria to simply not use her ability when looking at the deck (e.g. with Parallel Fates (2) which allows reordering), then maneuver a Prophecy of the End to a favorable position (either making sure she gets rid of two at a time with Psychic Sensitivity, or more generally, someone draws it and surges into a card they can handle, making the deck safe until the next reshuffle).
You know how many Prophecies you've eaten, the content of the encounter discard pile is open information and Gloria's should limit you to taking one Prophecy at a time, so you always know whether you are at risk and can plan accordingly.

Regardless, this is still a weakness that can disrupt Gloria's game plan in two significant ways:

1. Obviously, it stops Gloria from discarding dangerous encounter cards or removing them from circulation.
2. Less obviously, as discussed in AlexP's review, Gloria can only take one Prophecy under her if multiple are seen in one , so based on my literal interpretation of the rules, she would have to activate the "if you cannot" clause on the other copy (copies) of Prophecy and release cards into the encounter discard pile earlier than she intended.

On the other hand, a liberal application of the Silver Rule would invalidate everything I've just said, as it can be said that the "it must be chosen" text on each Prophecy takes precedence over Gloria's "choose 1 non-Elite card". In that case she will be forced to take all 3 Prophecies and be instantly defeated if they all show up in a single look.
A ruling may be in order here, but even if this worst-case scenario turns out to be true, Gloria can simply hold back her full Mythos-controlling power for a while until the first Prophecy has shown up and is safely in the discard pile.

Either way, the threat of the Prophecy is mostly intangible - what it can potential do to you stops you from going fully willy-nilly against the encounter deck. This is a design choice I found wonderfully thematic - if I had a nickel for every cosmic horror protagonist that got hurt/mad/traumatized for life/etc. after delving too deep I would be richer than Preston after all. And that's not mentioning the absolutely gorgeous card art, quite possibly my favorite among all released cards so far.

Rules ambiguity and gushing over themes aside, at the end of the day Gloria is a 5 4 Mystic with unparalleled control over the encounter deck, and Prophecy of the End doesn't really do much to change either. This is an interesting and well-designed weakness (certainly way more thought-provoking than Liber Omnium Finium, which in practice often turns out to be nothing more than a repeatable dead draw) that requires some playing around, but I think it's still quite undertuned compared to Gloria's absurd power level.

Am I wrong to understand that the Forced effect is resolved for each copy of Prophecy of the end ?

Meaning that if you look at 3 cards AND are very unlucky you can be defeated by a single use of her capacity. In scenarios with thin encounter decks (The Untamed Wilds for example), I would seriously consider not using Gloria's capacity until there is a copy of Prophecy of the end in the encounter discard.