When Gonti, Lord of Luxury enters the battlefield, look at the top four cards of target opponent's library, exile one of them face down, then put the rest on the bottom of that library in a random order. You may look at and cast that card for as long as it remains exiled, and you may spend mana as though it were mana of any type to cast that spell.
9/20/2016 You may look at and cast that card (and spend mana as though it were mana of any type to do so) even if Gonti leaves the battlefield. If another player gains control of Gonti, that player can’t look at or cast the card, and you still can.
9/20/2016 An effect that instructs you to “cast” a card doesn’t allow you to play lands.
9/20/2016 You pay the costs for the exiled card if you cast it. You may pay alternative costs such as emerge rather than the card’s mana cost.
9/20/2016 Gonti doesn’t change when you can cast the exiled card. For example, if you exile a creature card without flash, you can cast it only during your main phase when the stack is empty.
9/20/2016 Casting the card causes it to leave exile. You can’t cast it multiple times.
9/20/2016 If you leave the game, the card remains exiled face down indefinitely. No player may look at it.