Hellbent -
Entomb instead.
Being honest, I've never cast this card and have always though it was awful, but it probably plays better than it looks. Everyone will be scared of running it, since they've learned to respect
Hymn to Tourach.
Is that something for any red deck? If not, what decks want this?
Gamble is a weird card. I've played it in Constructed - Jund Depths (great) and an
Aggressive Mining brew (terrible) - so I feel kinda qualified to speak to its strengths but tbh i'm a scrub who knows nothing so caveat lector
Gamble is pretty strong. It's a powerful tutor that sometimes just goes ahead and punches you in the gut. That said, if there's one thing
Magic players are bad at, it's statistics. Pairing it with card draw, bouncelands, Loam (oh man, especially Loam), or even just slower, card-advantage-focused builds can mitigate its risks a bunch. Here's the thing about Gamble, though: Magic doesn't have a lot of random discard in it and so it's usually frustrating to lose something cool on a small personal level.
You can't top-deck Gamble and tutor your way out of the situation (unless you play from the graveyard, as you point out). At best you've got 50% odds of your card still being in your hand next turn, if you're still alive. That's an interesting dynamic shared by the top-of-library tutors, but at other times Gamble is sort of like Entomb, or sort of like Demonic Tutor. All pretty good cards, but the inconsistency means it's hard to evaluate, and most people will evaluate random discard incorrectly, overestimating the WCS. It's frustrating to have your game smoothing card not actually do that when you need it to, but it's not like a bunch of other cards would've helped either.
-one situation that's unlikely to come up in a lot of cubes (tragedy!) is that if you have two recursive cards in hand (say two witnesses or uh snapcaster yawg will) Gamble's drawback is entirely mitigated. This isn't something to build for but it's a nice way to give the card more obvious homes.
Red decks that want Gamble probably:
-play out of the graveyard at least a little bit (topdeck gamble, entomb and flashback Firebolt is mediocre but might be valid over some pass-the-turn lines)
-have some kind of card advantage or recursion scheme, if not necessarily an engine
-failing that, several redundant cards (often Big Cheatz decks but not always)
-have very impactful spells they're willing to take a risk on (some Painter (not Imperial Painter, which has a
better and more expensive tutor) builds run it to find combo pieces)
-run exciting situational lands
Seems like an okay list but it's still entirely possible nobody will ever draft it, because they're right, random discard is bad. It's just not quite as bad as they think it is, since you don't have to gamble until the late-game (or you can combo out early when you still have lands in hand) but it's still pretty bad.
e: my conclusion, re-reading this, is that we should all proxy and cube Imperial Recruiter.