1)
Why did He command him to prophesize about Gog? Also the previous Nevu'ah was about Gog!
1.
Malbim: The previous Nevu'ah, Nevi'im before Yechezkel already foretold it ? "ha'Atah Hu Asher Dibarti b'Yamim Kadmonim" (17). The coming Nevu'ah I designated only for you.
2)
What do we learn from "Hineni Elecha Gog"?
1.
Malbim: After this fall, he will be strengthened a second time and gather his camps a second time against Yerushalayim.