there is one card that allows to play treasure cards during the action phase, and it's special and confusing and allows for all kinds of tricks and we probably dont need another one.
Black Market is special because it lets you buy kingdom cards that aren't in the supply and confusing because it does stuff that isn't written on the card. Castle Gate does none of that, and allowing all kinds of tricks is a good thing as long as those tricks are balanced or rare enough.
Castle Gate/Tactician is not a particularly strong combo, it's probably weaker than Tactician/Scavenger, which is not even a thing really, and the same is true for draw-to-x. It has a special kind of positive interaction with Menagerie, which is a strong card on its own, but it doesn't sound like anything to worry about. It also has a special kind of positive interaction with Poor House, but it only works in situations where Poor House sucks anyway.
To me, it doesn't look like you "have to" limit it to Actions at all. However, there is an actual problem with not limiting: a single-card no-benefit trasher that can be used as a pseudo-Beggar in the late game could be weak but balanced at $2 on its own (comparable to Stonemason without overpay and probably better). This card does that and much more — it turns multiple mediocre turns into one strong turn and a number of weaker ones, connects your cards that you want to connect (except it doesn't work for Treasure Maps), and functions as a splitter.
Limiting it, though, creates a new problem, which is being a dead card in a hand with no Actions. It's not a problem for Throne Room, but I think it might be a bit of a problem here, since Castle Gate is not suitable for decks that are already consistent while Throne Room is.