What is the ruling on the actions of a father who consistently fumigates my grandmother's house and recites the Qur’an for her after her death, and prepares her bed, pillow, a glass of water, and a copy of the Qur’an for her, believing that she visits and sees them? And is what I have mentioned about the father's actions considered backbiting (Gheebah)?

The soul has actions both in life and after death, known only to Allah. It is not permissible to claim knowledge of any of them without legitimate religious proof. Your father's actions, such as preparing your grandmother's bed and fetching water for her, are an innovation (bid'ah) and are not based on knowledge, because the deceased does not benefit from such things. Your duty is to advise your father to abandon this habit and replace it with supplication (du'a) for your grandmother and giving charity on her behalf.