Why should he say so to Gog?
Malbim: There are two Nevu'os here. (a) He will do evil to Yisrael in the future, at the time of Churban Bayis Sheni and Galus. About this, it says "Hinavei Ben Adam." (b) He will receive his punishment on Harei Yisrael ? "v'Amarta l'Gog."
When is "ba'Yom ha'Hu"?
Malbim: The time of Churban Bayis Sheni, when Yisrael dwelled confidently in their land.
Why does it say "Teda"?
Targum Yonasan: You will know the punishments of My Gevurah. Rashi ? i.e. you will know and recognize the strength in which they trust.
Radak: When you will know that they dwell confidently in their land, you will come from your place.
Malbim: You will know what you will do to them.
