It is parenthetical.  Just as we might write "the leader of the United States - Pres. Donald Trump - is announcing..."  Its usage is originally from Ezek. 38 who was sent to tell Gog - the leader of the land of Magog - that YHWH was against him.  Gog was a wicked king, leader of idolatrous people.  The prophesy of Ezekiel that Gog and Magog would come against Israel in their latter days (See Gen. 49:1-10) was the idolatrous nations which came against Israel in the prophesy of Rev. 20: 7-8. Young's Literal Translation has Ezek. 38:2 -

>"2 `Son of man, set thy face unto Gog, of the land of Magog, prince of Rosh, Meshech, and Tubal, and prophesy concerning him,  3 and thou hast said: Thus saith the Lord Jehovah: Lo, I [am] against thee, O Gog, Prince of Rosh, Meshech, and Tubal,"

Gog and Magog stood for the idolatrous nations which surrounded Judea on all sides, or "the four corners of the earth".  Which "earth" was Judea and Palestine, from OT usage where "earth" was most often synonymous with Israel.  As "earth" was Judea, the "sea" were the idolatrous nations which surrounded the "earth" of Judea as water surrounds the land.