Gruesome Menagerie — ruling 1

Gruesome Menagerie doesn't target the cards to return. You choose them while it's resolving. No players may take actions between the time you make each choice and the time you return them to the battlefield.

Gruesome Menagerie — ruling 2

If a card in your graveyard has {X} in its mana cost, X is considered to be 0.

Gruesome Menagerie — ruling 3

If you don't have a creature card in your graveyard with one mana value, you just continue on to the next.

Gruesome Menagerie — ruling 4

All of the returned cards enter the battlefield at the same time.

Gruesome Menagerie : règles, fonctionnement et interactions (MTG)

Gruesome Menagerie is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Choose a creature card with mana value 1 in your graveyard, then do the same for creature cards with mana value 2 and 3. Return those cards to the battlefield.

How it works : Gruesome Menagerie

Original English text — Wizards of the Coast / Scryfall

#1October 5, 2018· Wizards of the Coast
Gruesome Menagerie doesn't target the cards to return. You choose them while it's resolving. No players may take actions between the time you make each choice and the time you return them to the battlefield.
No other card is named in this ruling.
#2October 5, 2018· Wizards of the Coast
If a card in your graveyard has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#3October 5, 2018· Wizards of the Coast
If you don't have a creature card in your graveyard with one mana value, you just continue on to the next.
No other card is named in this ruling.
#4October 5, 2018· Wizards of the Coast
All of the returned cards enter the battlefield at the same time.
No other card is named in this ruling.