All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
Based on what you have mentioned, the deceased did not gift the land to his wife or to any other person before his death, and he did not leave any clear will about this as far as we understood from the question. If we presume that he had made a will of this land to your mother, then it is a will for an heir, and so, it is not effective unless all the other heirs approve of it.
Just having the intention or promising to gift the land to her is of no effect and it should not be taken into account and it does not result in anything. Therefore, the land is part of the estate that should be divided between all the heirs of the deceased, each according to his legal share. Among the heirs is your grandmother (the mother of your father); and her share is one-sixth of the estate.
Allaah Knows best.