It's not a synonym for death, because it comes after death, and it's not a synonym for burial in an ancestral grave (i.e. for being literally gathered to one's ancestors), because the report of burial comes after being gathered to his people. In Jacob's case, his burial followed being gathered to his people by a long period. So, Sarna says, "It would seem, therefore, that the existence of this idiom . . . testifies to a belief that, despite his mortality and perishability, man possesses an immortal element that survives the loss of life. Death is looked upon as a transition to an afterlife where one is united with one's ancestors. This interpretation contradicts the widespread, but apparently erroneous, view that such a notion is unknown in Israel until later times." (JPS Torah Commentary, Genesis, p. 174)

This prooftext for intimations of immortality in the Torah comes before the one that Messiah Yeshua cites, Exodus 3:6, 15, "But concerning the resurrection of the dead, have you not read what was spoken to you by God, saying, ‘I am the God of Abraham, the God of Isaac, and the God of Jacob’? God is not the God of the dead, but of the living." (Mat. 22:31-32)

So our belief in resurrection, life after death, is firmly rooted in Torah itself. I'm wondering how this connects with enthusiasm, or zerizut. It strikes me that we can live this life with more adventure and less fear, more readiness to risk, and far less regret, when we know that there's life beyond this life.