What is the significance of them not knowing Hashem's intents?
Radak: They do not know why He aroused them to come to Yerushalayim. They think that it is to plunder its spoils and destroy it. They do not realize that He aroused them ? "v'Ha'alisicha mi'Yarkesei Tzafon va'Havi'osicha Al Harei Yisrael" (Yechezkel 39:2).
Why does it add "v'Lo Hevinu Atzaso"?
Malbim: Hashem's Etzah is the desired purpose of His deeds. His Machshevos are the means in order to prepare the Etzah (purpose). The gathering is in order to judge them for their deeds, and there they will understand the true creed via the signs and wonders that they will see. The survivors will return to the true Emunah.
What is the comparison to "Amir Gorenah"?
Rashi: In the end, Hashem will gather them to Emek Yehoshafat and around Yerushalayim for war, just like they gather Amir (bundles of sheaves) to thresh them. Also the army of Gog and Magog were gathered for their evil.
Malbim: The Resha'im will be eradicated like chaff is blown from the granary. Just like one gathers sheaves to the granary in order to remove the chaff and shell and the grain will remain, so He will thresh them in war of Gog and Magog and remove the chaff (eradicated the Resha'im who did evil ba'Kodesh) and clarify the grain. He will separate from them converts who recognize the true creed.
