Is the father entitled to claim his share from the inheritance of his deceased grandfather, given that his grandfather had gifted him a piece of land during his lifetime and he had disposed of it? And are the heirs entitled to revoke this gift?

Your father has the right to claim his share of his father's inheritance, regardless of any prior gift from the father to him. The heirs are not permitted to deprive him of it or consider the gift as inheritance. If the gift was made during a non-fatal illness and your father took possession of it before your grandfather's death, then it is valid and is not to be returned to the estate. It is recommended that your father return it to the estate and have it distributed according to Islamic law. In case of dispute, the matter must be referred to the Sharia court.