Cataclysmic Gearhulk — ruling 1

First the player whose turn it is chooses permanents they control, then each other player in turn order does the same. Each player will know the choices made by players who chose before them. All of the unchosen nonland permanents are then sacrificed simultaneously.

Cataclysmic Gearhulk — ruling 2

A permanent with more than one type may be chosen as either of the types or both. For example, an artifact creature may be chosen as the artifact, the creature, or both. Choosing the same permanent twice this way has the same result as choosing it once.

Cataclysmic Gearhulk — ruling 3

Cataclysmic Gearhulk will be sacrificed to its own ability if you don't choose it as your artifact or your creature.

Cataclysmic Gearhulk — ruling 4

Lands can't be chosen and won't be sacrificed, even if they have other types (such as an artifact land).

Cataclysmic Gearhulk — ruling 5

If you don't control any permanents of one of the types, you'll still choose the permanents of the types you do control.

Cataclysmic Gearhulk : règles, fonctionnement et interactions (MTG)

Cataclysmic Gearhulk 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

Vigilance When this creature enters, each player chooses an artifact, a creature, an enchantment, and a planeswalker from among the nonland permanents they control, then sacrifices the rest.

How it works : Cataclysmic Gearhulk

Original English text — Wizards of the Coast / Scryfall

#1September 20, 2016· Wizards of the Coast
First the player whose turn it is chooses permanents they control, then each other player in turn order does the same. Each player will know the choices made by players who chose before them. All of the unchosen nonland permanents are then sacrificed simultaneously.
No other card is named in this ruling.
#2September 20, 2016· Wizards of the Coast
A permanent with more than one type may be chosen as either of the types or both. For example, an artifact creature may be chosen as the artifact, the creature, or both. Choosing the same permanent twice this way has the same result as choosing it once.
No other card is named in this ruling.
#3September 20, 2016· Wizards of the Coast
Cataclysmic Gearhulk will be sacrificed to its own ability if you don't choose it as your artifact or your creature.
No other card is named in this ruling.
#4September 20, 2016· Wizards of the Coast
Lands can't be chosen and won't be sacrificed, even if they have other types (such as an artifact land).
No other card is named in this ruling.
#5September 20, 2016· Wizards of the Coast
If you don't control any permanents of one of the types, you'll still choose the permanents of the types you do control.
No other card is named in this ruling.