When an opponent casts an instant spell, if Hidden Gibbons is an enchantment, Hidden Gibbons becomes a 4/4 Ape creature.
10/4/2004 When it turns into a creature, it is no longer an enchantment.
10/4/2004 It changes into a creature even if the spell is countered.
8/1/2008 The ability triggers when the spell is cast, so it becomes a creature before the spell resolves.
8/1/2008 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.