What does "gain" mean?
To gain a card is to take it from the lineup and move it to your discard pile. From page 9 of the rulebook:


  Gaining Cards
  
  
    When a card tells a player to gain a particular card
    or a card of your choice, that card is taken and
    immediately placed in that player’s discard pile at no
    additional cost, unless otherwise directed by the card.