Gyrus, Waker of Corpses

• WWizards of the Coast2018-07-13
  Official Ruling
  Any enters-the-battlefield abilities of the copied creature card will trigger when the token enters the battlefield. Any "as [this creature card] enters the battlefield" or "[this creature card] enters the battlefield with" abilities of the chosen creature card will also work.
• WWizards of the Coast2018-07-13
  Official Ruling
  Although the token is attacking, it was never declared as an attacking creature (for the purposes of abilities that trigger whenever a creature attacks, for example).
• WWizards of the Coast2018-07-13
  Official Ruling
  If the copied creature card has in its mana cost, X is considered to be 0.
• WWizards of the Coast2018-07-13
  Official Ruling
  The amount of mana you spent to cast Gyrus is usually equal to its mana value, normally 3 plus the value chosen for X. However, you also include any additional costs you pay, including the commander tax.
• WWizards of the Coast2018-07-13
  Official Ruling
  Gyrus's token copies exactly what was printed on the original creature card and nothing else. It doesn't copy any effects that modified it before it was put into your graveyard.
• WWizards of the Coast2018-07-13
  Official Ruling
  You choose which player or planeswalker the token is attacking. It doesn't have to be attacking the same player or planeswalker that Gyrus is attacking.
• WWizards of the Coast2018-07-13
  Official Ruling
  If another creature enters the battlefield as a copy of Gyrus, consider how much mana was spent to cast that creature to determine how many +1/+1 counters it enters with.
• WWizards of the Coast2018-07-13
  Official Ruling
  If Gyrus enters the battlefield without being cast, then no mana was spent to cast it. It enters the battlefield without any +1/+1 counters. If no other effects are increasing its toughness at that time, it will be put into its owner's graveyard as a state-based action.

Rulings via Scryfall