The "grapes" are those who chose not to believe in Jesus Christ as Lord and Savior, both Jew and Gentile. The tribulation of God's wrath on earth and hell to follow is the winepress of wrath.
As you know, the song is an awesome unity of the reality of war intersecting with the war of wars always running underneath, the battle of Good and evil. At the end-God wins and evil that was defeated at the cross is realized when all realize Jesus Christ is Lord: the evil ones are slain and thrown into the eternal lake of fire.