Are we entitled to ownership of the gifted land, and is a daughter entitled to revoke her father's gift after his death and build houses on it, and are we permitted to protect this land?

If you took possession of the land during the lifetime of the donor, then it is your property, and no one has the right to seize it. Your protection of it is like protecting your own wealth, and whoever dies defending their wealth is a martyr. The absence of official documents does not change the religious ruling. However, if the donor died before you took possession of the land, then the gift is void, and the land becomes the right of their heirs.