Saying that you can't gain as per the wording you have used WILL result in direct contradiction between two cards. Why should your card have default precedence?
The "do as much as you can" rule doesn't currently apply to this situation because the interaction doesn't exist in this capacity (though it does to an extent in that Develop or any other trasher that uses "exact" gains nothing if the appropriate priced card doesn't exist). I'm not arguing that the mechanic should exist, but the game really is well equipped to handle it. If I thought there was real potential rules confusion, I'd agree with your point, but I don't believe it to actually exist any more than other novice rules understandings.
I don't follow this at all. Dominion is conspicuously NOT equipped to handle conflicts like this. I say conspicuous because of how much deliberate care there is in the wording of the cards to avoid ever encountering such a conflict, no matter how unlikely or contrived a situation you'd need.
To reiterate, if one card says "gain X" and card X says "you can't gain this," then there is NO WAY to resolve the situation. There is nothing inherent in the wordings or context of either card to suggest that one or the other should take precedence. You have to invent a new rule for the game overall -- not just a rule on a card somewhere but a rule governing the mechanics and terminology of the entire game -- that would describe how you would resolve such a conflict.
The reason "can't" works in the three cards that use it:
(1) Outpost's "can't" only qualifies what IT does and does not impose any restrictions on what other cards can do. It is significant that it doesn't say "You can't play this if it would cause you to take more than two consecutive turns," because then it would conflict with a card that told you to play it. (Golem turning up two copies, for instance.)
(2) Grand Market says you can't
buy it with Copper in play, and that's okay because buying is always something the player is free to do. No rule -- either in the game or on another card -- tell you to buy something. Therefore, if no rule forces you to do it, it's fine if a card forces you not to. Very significantly, Grand Market does NOT say you can't
gain it with Copper in play, because there are tons of other cards that force you to gain things.
(3) Contraband, again, only says "can't" with respect to buying cards, not gaining them. I can play Contraband, be forbidden Provinces, then play a Horn of Plenty and gain a Province. But if it said "gain," then what do I do if Coppers and Curses are out, Estates are the only $2 cards out, Contraband forbids me Estates, and then I play a Horn of Plenty worth $2? This is a crazy edge case, and yet Donald very conspicuously wrote Contraband to avoid this kind of conflict. I don't know why you'd think a fan card exhibiting less care would be acceptable to Dominion players at large.
It's all well and good to say "Can't is simple -- can't means can't" and trick yourself into thinking it's all perfectly clear. But you could just as easily say "Gain is simple -- gain means gain" and trick yourself the other way.