What does Abraham’s being “gathered to his people” mean?
The question arises from Genesis 25:8: “Then Abraham gave up the ghost, and died in a good old age, an old man, and full of years; and was gathered to his people.” He had reached the ripe old age of 175 years when he died, and he was buried by his sons in the cave of Machpelah. The expression is repeated again relative to Ishmael (Gen 25:17), Isaac (35:29) and Jacob (49:29,33).
Abraham had been told by God earlier that he would “go to [his] fathers in peace [and] be buried in a good old age” (15:15). Later, Joshua and his generation would be “gathered unto their fathers” (Jdg 2:10).
Clearly, these similar expressions are taking us in thought beyond death and the grave to the dwelling place of the departed, which is known in the Old Testament as “Sheol,” a Hebrew word meaning “the depths.” There was an anticipated reunion there with those believers who had previously died. The continuance of existence beyond the grave is assumed, and the writer to the Hebrews informs us that these OT saints “died in faith, not having received the promises …. But now they desire a better country, that is, an heavenly: wherefore God is not ashamed to be called their God: for he hath prepared for them a city” (Heb 11:13,16).
The psalmist looked forward to being delivered from the power of Sheol. “But God will redeem my soul from the power of the grave [sheol]: for he shall receive me. Selah” (Psa 49:15). The hope of resurrection was known to OT saints.
So we would conclude that Abraham left this life for an ongoing existence in a place where he would meet and dwell with believers who had previously died. Death for them was not final, and there will be resurrection in a future day when the promises of God will be fulfilled. “Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years” (Rev 20:6).
I believe that Abraham and all the saints who have died in every previous dispensation will have part in this first resurrection, and in glorified bodies have a place in the heavenly city. They are all redeemed by the blood of the Lamb and are in eternal relationship to the Lamb, the Redeemer, the One who has power over death and hell.
When will this resurrection take place? Revelation 20:4 is speaking of the resurrection of tribulation martyrs, who are not part of the Church. The Church will have been removed from the earth at the Rapture and thus the dead “in Christ” will have already been raised. There is a second stage in the first resurrection at the end of the tribulation, and I take it that the OT saints are raised at this point and assessed as to reward (Mat 25:19) to enter their role in the Kingdom of our Lord Jesus Christ in that heavenly city. Daniel would also stand in his lot at that point at the end of the tribulation (Dan 12:13).
 All Scripture quotations in this Q&A are from the KJV.