Is the drying up of the Sea of Galilee a sign of the Mahdi's coming, and is it a result of Gog and Magog drinking its water?

The drying up of Lake Tiberias (Sea of Galilee) is not a sign of the Mahdi's advent. Rather, it occurs after the Mahdi's era has passed, and it will be due to Gog and Magog drinking its water after Jesus (peace be upon him) descends and kills the Dajjal, as stated in Sahih Muslim.