Opal Gargoyle

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

It triggers when the spell is cast, which means it becomes a creature before that spell resolves.

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

A "creature spell" is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.

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.