Is the grandmother's action of writing the ownership of the apartment to one of her granddaughters, without the knowledge of the other heirs, considered a valid gift or will according to Islamic law, or is it a circumvention of inheritance laws?

If the grandmother transferred ownership of the apartment to her granddaughter while the latter was in good health and of sound mind, and the granddaughter took legitimate possession of the apartment during her grandmother's lifetime, then the gift is valid, and the apartment is the private property of the granddaughter. The remaining heirs have no right to it. However, if there is no proof, or if the apartment was not taken possession of during the grandmother's lifetime, then the apartment is part of the inheritance like any other asset, unless the heirs relinquish their share. It is advisable to consult the Sharia courts to ascertain the details of the matter.