The effect "Unaffected by Spell/Trap effects. Your opponent's monsters cannot activate their effects during the Battle Phase. " is a Continuous Effect.
The effect "Unaffected by Spell/Trap effects." is applied as long as "Chaos Ancient Gear Giant" is face-up in the Monster Zone. (It is unaffected by your Spell/Trap effects and your opponent's Spell/Trap effects. It is also applied during the Damage Step.) Also, the effect "Your opponent's monsters cannot activate their effects during the Battle Phase." is only applied during your Battle Phase and your opponent's Battle Phase.
The effect " This card can attack all monsters your opponent controls, once each. If this card attacks a Defense Position monster, inflict piercing battle damage to your opponent." is a Continuous Effect.
Even if "Chaos Ancient Gear Giant" is Fusion Summoned by the effect of "Power Bond", the resolution of "and it gains ATK equal to its original ATK. During the End Phase of the turn this card is activated, you take damage equal to the amount of ATK gained from this effect at the time of the Summon." is applied normally. (Its ATK is increased from 4500 to 9000, and you take 4500 damage during the End Phase.)