Revelation 20:7,8

7  And when the thousand years are expired, Satan shall be loosed out of his prison,

8  And shall go out to deceive the nations which are in the four quarters of the earth, Gog and Magog, to gather them together to battle: the number of whom is as the sand of the sea.

When the thousand years are over, Satan will be loosed out of the prison to deceive the nations (ethnic groups) to gather them together to battle. Some think this is the war of Ezekiel Chapter s 38 and 39 because of Gog and Magog being mentioned. It cannot be for many reasons.  One reason is that  after the war of Ezekiel 38, 39 people are buried and weapons are burned.  However, when these come against the camp of the saints they are consumed by fire. Immediately afterward, the white throne judgment takes place in heaven.

Descendants of the tribes of Gog and Magog may be the most prominent on the earth at that time or God may just name locations on the earth after these prominent tribes. Gog and Magog may just be another way of saying near and far, or north and south or something similar. 

Notice the number is as the sand of the sea. Apparently, under the near perfect conditions on earth during the thousand-year reign, more billions will be born than in all ages of the earth put together.  The prior six thousand years on earth were primarily to establish a government of kings and priests for this sabbatical millennial age.  The kings and priests will rule over the great harvest of souls that God intended when He told Adam to be fruitful and multiply and subdue the earth.