Hidden Gibbons — ruling 1

The ability triggers when the spell is cast, so it becomes a creature before the spell resolves.

Hidden Gibbons — ruling 2

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Hidden Gibbons — ruling 3

When it turns into a creature, it is no longer an enchantment.

Hidden Gibbons — ruling 4

It changes into a creature even if the spell is countered.

Hidden Gibbons : règles, fonctionnement et interactions (MTG)

Hidden Gibbons 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

When an opponent casts an instant spell, if this permanent is an enchantment, it becomes a 4/4 Ape creature.

How it works : Hidden Gibbons

Original English text — Wizards of the Coast / Scryfall

#1August 1, 2008· Wizards of the Coast
The ability triggers when the spell is cast, so it becomes a creature before the spell resolves.
No other card is named in this ruling.
#2August 1, 2008· Wizards of the Coast
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
When it turns into a creature, it is no longer an enchantment.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
It changes into a creature even if the spell is countered.
No other card is named in this ruling.