Gyruda, Doom of Depths

• WOTC Official Reminder Textofficial rules
  Companion — If your starting deck meets this condition, you may begin the game with this card outside your starting hand.

• WWizards of the Coast2020-04-17
  Official Ruling
  If a replacement effect causes a player to exile the top four cards of their library instead of putting them into their graveyard as Gyruda's triggered ability resolves, the creature card you choose may be one of those cards in exile.
• WWizards of the Coast2020-04-17
  Official Ruling
  If you reveal a companion outside the game, for as long as it remains there, you may pay any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and behaves like any other card you've brought into the game. For example, if it's discard, countered, or destroyed, it's put into your graveyard, remaining in the game. This is a change from previous rules.
• WWizards of the Coast2020-04-17
  Official Ruling
  If more than one player wishes to reveal a companion, the starting player does so first, and players proceed in turn order. Once a player has chosen not to reveal a companion, that player can't change their mind.
• WWizards of the Coast2020-04-17
  Official Ruling
  The companion's other abilities apply only if the creature is on the battlefield. They have no effect while the companion is outside the game.
• WWizards of the Coast2020-04-17
  Official Ruling
  You may have one companion in the Commander variant. Your deck, including your commander, must meet its companion requirement. Your companion is not one of your one hundred cards.
• WWizards of the Coast2020-04-17
  Official Ruling
  The companion ability has no effect if the card is in your starting deck and creates no restriction on putting a card with a companion ability into your starting deck. For example, Zirda may be in your starting deck even if your other permanent cards don't all have activated abilities.
• WWizards of the Coast2020-04-17
  Official Ruling
  Paying to put your companion into your hand is a special action. It doesn't use the stack and players can't respond to it. Once you take this action, you may cast that card if it's legal to do so before any other player can take actions.
• WWizards of the Coast2020-04-17
  Official Ruling
  Your companion begins the game outside the game. In tournament play, this means your sideboard. In casual play, it's simply a card you own that's not in your starting deck.
• WWizards of the Coast2020-04-17
  Official Ruling
  If a card in a player's graveyard has in its mana cost, X is considered to be 0.
• WWizards of the Coast2020-04-17
  Official Ruling
  The requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard.
• WWizards of the Coast2020-04-17
  Official Ruling
  Before shuffling your deck to become your library, you may reveal one card from outside the game to be your companion if your starting deck meets the requirements of the companion ability. You can't reveal more than one. It remains revealed outside the game as the game begins.
• WWizards of the Coast2020-04-17
  Official Ruling
  A card with mana value 0 has an even mana value.
• WWizards of the Coast2020-06-01
  Official Ruling
  Paying to put your companion into your hand is a special action. It doesn't use the stack and players can't respond to it. Once you take this action, you may cast that card if it's legal to do so before any other player can take actions.
• WWizards of the Coast2020-06-01
  Official Ruling
  If you reveal a companion outside the game, for as long as it remains there, you may pay any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and behaves like any other card you've brought into the game. For example, if it's discarded, countered, or destroyed, it's put into your graveyard, remaining in the game. This is a change from previous rules.

Rulings via Scryfall