A gain is simply getting a card into your deck in whatever way it happens*, whether Ironworks, buying a card, or even gaining a Curse from an opponent's Witch. Maybe explaining it that way would help.
*Except for having it passed to you by Masquerade, or exchanging a Traveller, or when you trash a Fortress and it gets returned to your hand.
Okay, getting a new card into your deck then. Travellers are simply the same card evolving in a real sense, and Masquerade is passing old cards. Fortress is already in your deck.
Each Traveller upgrade is a new card, even if the thematic idea is that it's the same one. When Masquerade passes you a card, it
is new to
your deck. When Fortress is trashed, it is briefly no longer in your deck. If you don't consider Fortress a new card from the trash, then what about when you gain from the trash via Graverobber or Rogue? Those cards aren't "new" either then.
We take these terms for granted, but they can certainly be confusing for new players. FWIW, I'd explain it like this:
- Buying is when you use a Buy in the Buy phase. After you buy a card in the Supply, you gain it.
- Gaining also happens when a card is played that tells you to gain a card. It actually says "gain" right on it. When you do this, you gain the card without buying it.
And then I'd address any issues for a particular kingdom, like Trader or Black Market. I wouldn't go into details unless they are relevant for the game. Events are another wrinkle (they are things you buy but you don't gain; for in-game terminology they aren't even "cards").