Is the giant of castelnau real?

Yes the discovery was real. Check out the French Science journal La Nature Vol. 18, 1890 issue 888. pg 11-12. The discovery is described in detail in that publication by the Anthropologist Georges Vacher de Lapouge, who found the bones at the bottom of an ancient Bronze age tumulus near Castelnau Le-Lez, France in the winter of 1890.