Gatekeeper: This seems like a really good, well designed card, and is better than Secret Chamber.
Replica: Pretty weak and risky. Gaining the new treasure to hand would be a start. An ability that let you put it on the Tavern mat to watch for treasure gains hand after hand and never whiff seems possible.
If you're new to Dominion, Gold might be purchased somewhat more frequently in your games compared with expert play. 5$ cards are often better and they're also only worse by a slim margin lots of the time. So buying a 5$ action with a 6$ hand would be an easy way to devastate a player that bought Replica.
Wayfarer: It's a nice idea, but +action cards that don't give +1 card are generally fighting uphill from the getgo because it can be so challenging to get 2 terminals into the same hand reliably. Since these require eachother to be drawn together before they even become empty Universities, they are taxing a requirement that was already an issue.
Maybe the idea here is to use Market and Woodcutters, but they don't compose that much of the cardpool.
It's still a neat card though. Why don't you just give it +card and price it at 4$? It wouldn't be an Ironmonger at all.
Antiquarian: Wow, what a cool card. Very thematic. Very printable as is.
Offertory: The comparison to Ironworks here is troubling. One of Ironworks's options is +1$, gain a silver. This +1 Card, +1 Action, -2$, gain a silver. That's net 0 vanilla bonuses instead of net 1. And Ironworks is free to do other things, whereas this can't do anything else (in kingdoms without an extra treasure, 95+% of them). Of course it's not always bad for a 3$ card to be worse than a 4$ card. But they should be pretty close, and this is a gulf. Workshop doesn't even make this card look great.
Rainbow: Very cute and probably fine. The Vorthos in me is mad though, a Rainbow is about diversity.
Reclamation: I suspect this doesn't do what you want it to. When a card tells you to do something to N objects, and you have M objects, 0<M<N, you do that thing to those M options to do as much as you can. We see this with 1 card hands getting Torturured in Dominion and see it in other cases in other games. So when you are instructed to pick 3 treasures from the trash, it's pretty likely there are only 1 or 2, and you'll still pick them. You'd need a wording like "pick a set of 3 treasures from the trash" or need to throw the word "exactly" into there. An alternative choice that game designers often use when the "do as much as you can" rule isn't working in a way that serves their design is to just clearly arrange the effects so that it does. Here you would have players return treasures to the supply before gaining, which would make the requirement that 3 treasures are in the bin you seemed to intend a requirement for the card's second mode to be useful rather than a requirement for it to be legal.
Restorer: Probably fine.
Ambush: Seems fun. This is probably ok at two or three cost. Since Durations miss the reshuffle twice as often, making it easier to buy multiples could add some fun factor in keeping village strategies cohesive in the odd game where this is the premiere village.
Craftsmen: The top part seems too powerful for the bottom to do very much at all. Some games the bottom isn't doing very much at all, but this seems to blank junkers pretty hard.
I'm not sure the rules truly support grouping several gains together and applying a replacement effect across them. When you buy a cache, can you reveal this and specify that the 2 copper gains are being replaced but the imminent cache gain is not? Of course, for some players they'll happily use house rules and understandings just for their playgroup and not care about it, but your cards have been pretty clean thus far ruleswise.
Cursed Gold: I'm gonna jump on the bandwagon of saying this card is just plain cool.
Furnace: This thing is really neat. It is a nonterminal Mine you've costed at 4$ though. Mine would be totally fine at 4$, people have agreed on that in the past. Nonterminal Mine. With other upsides? Concern.
Changing the cost messes with the crazy cool top and bottom half interaction. So does weakening the effect. So does removing +1 Action. I think you should just add a penalty to it to balance it. Embassy clause, giving opponents VP, -1$ to the effect. But man this card is going places.
You could tack the drawback onto the below the line effect to be penalty when you buy it vs. the alternative of upside when you gain it some other way, so it feels like you haven't added that much noise.
Gold Digger's Village: I love it. Stables is one of my favorite cards, this brings that experience to a village. This looks safe at 3$, it's not strictly better than Village, and the primary risk of a 3$ card vs. a 4$ card is usually people acquiring too many, and these antistack, thirsting for the same resource. You should be fine. It's definitely not as lethal as Fishing Village's 3$ status.
Bribe: This is a great design, one of those cards that changes the landscape of cards in which it appears rather than being a new card in the kingdom per se.
Cathedral: This is cool. Comparison to Embassy might raise light fears it's too strong.
Collector: This is cool. Cleverly balanced.
Foundry: I like this. I think it might could stand a +1$. Compare to Trading Post.
Greed: I don't have much to say on this one.
Money Laundering: The only other terminal silver 5$ attack that can whiff entirely this often is Rogue and Rogue feels pretty bad for it. Was the attacker supposed to choose, here? As written the defender chooses and that's lots of whiffing. Maybe put the second gained copper into hand, not the first. So it's a 5$ Cutpurse with upside.
Stolen Goods: I love this card. It's so clever. Because those limitations are absolutely the things you need to do eventually, and this card will so nastily trap so many newbies, it's just glorious.
It can be a 4$ card, I think. It's strictly worse than Soothsayer, mostly. If you play a Soothsayer, you gain the most expensive nonvictory card, maybe also get something else, and attack the enemy player. The drawback of this turning into a Curse in endgame is a big deal. It seems a lot more fascinating at 4$ and less dependent on gainers in the kingdom.
Treasurer: I said Stables was one of my favorite cards, so how do you think I'm gonna feel about a strictly better stables with this many upsides over it?
Investor: Needs work. Making this card good is a lot of hoops you're jumping through.
Mountains: The below the line text seems unrelated and unmotivated, but I guess that's not so bad. This is kind of on the strong side, if you split the pile these are 4 point fairgrounds, which people have to put in a little work to get and are usually willing to do the work to get, whereas this doesn't require any work. It doesn't really ruin a game to have a dominating VP option that's not Province, though, Nobles and Colony don't ruin games either.
You might want to increase the impact of the second ability by 1, if you Trade Route a Silver instead of an Estate because you want to make use of Mountain's special mechanic, you don't actually get closer to buying a Mountain. That doesn't feel great.
Old Gem: This card is pretty cool. It won't be super rare for it to be the only nonterminal +buy on the board, and then its unique mechanic will be affecting the game.
All in all I really liked the vast majority of these cards, and OP, whose screenname I don't remember, is probably one of my favorite fan card designers now. I'm surprised he hasn't played with all the cards yet. But I hope he does, at least after ShuffleIT hits.
