There are two forms of salvation that take place in regards to the Great Tribulation. The eternal salvation of souls and the salvation of flesh.
All of those that enter the Millennial Reign of Christ upon the earth, in their flesh, accepted Christ through faith during the Great Tribulation, are saved both spiritually and in the flesh. All the unbelieving, that are in the flesh at the Lord's return, are slain. Those that accept Christ, during the Great Tribulation, shall come from all nations.
Many that accept Christ from all nations, during the Great Tribulation, shall be martyred by the Antichrist and his followers. Thereby, the salvation of their souls only.
Those surviving in the flesh shall repopulate the earth during Christ's Millennial Reign. Born into the sin-cursed flesh, the same as we have been. But absent the accentuater of sin, Satan, that we have dealt with, who is bound unable to accentuate sin upon the earth, until his release after the Millennial Reign.
The key to some Israelites and some people from all nations of the earth accepting Christ, during the Great Tribulation, is the depth of God's great love towards man.
The Great Tribulation is a time in which God and His adversary both display clearly seen supernatural power in their war for the souls of men. It's culmination is the casting down of the Antichrist's brief reign from Jerusalem and the onset of the Lord's righteous reign from Jerusalem. The fact that souls are saved from across the earth, and that 144,000 Jewish missionaries are utilized, during the time where the adversary is allowed to unleash his full arsenal, glorifies God and testifies of His faithfulness to His covenants. Israel has been targeted for extermination by God's adversary from the moment that God chose them.
Satan is afforded one last stab at Israel when he is released following the Millennial Reign of Christ. Satan has perpetually hated Israel. Israel is the apple of God's eye. A remnant of Israel shall be saved during the Great Tribulation and they shall be faithful servants of the Lord from then on. Fulfilling Ezekiel's prophecy.