Is the gift by a grandfather of a plot of land to his daughter’s son, who built a house on it during the grandfather’s lifetime, Islamically valid after the grandfather’s death, especially given that the heirs are displeased with this gift?

If a grandfather granted a plot of land to his grandson during his lifetime, and the grandson took possession of it and built upon it, then the gift is complete, and the land becomes the property of the grandson. The grandfather's heirs have no right to it after his death, whether they approve of the gift or not. This is further confirmed by the fact that the principle of equal distribution in gifts applies to children of the same degree, and grandchildren are not included in this ruling.