قَالَ الصَّبِيُّ: «غاق» — The boy said, “ghāq.”
قَالَ: past verb built on fatḥ.
الصَّبِيُّ: nominative subject with ḍammah.
«غاق»: the imitated sound. The quoted utterance functions as the reported speech in the accusative position.
Do not invent an accusative ending on the sound-word itself simply because the whole quotation fills the object position.