This phrase is used a number of times in the Old Testament at the time of someone’s death. I believe that I can establish that it means something more than being buried in the family tomb.
Gen 25:8 And 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.
Abraham had lived a long time away from his ancestral home and burying grounds. Only Sarah’s bones waited him in this world. Abraham went somewhere to be with someone, not just any someone, but those (people is plural) who were HIS people, dare we say, people of faith.
Gen 35:29 And Isaac gave up the ghost, and died, and was gathered unto his people, old and full of days: and Esau and Jacob his sons buried him.
Clearly, the gathering and the burying were distinct.
Gen 49:33 When Jacob finished commanding his sons, he drew up his feet into the bed and breathed his last and was gathered to his people.
In a clear distinction, dying, i.e., breathing his last, is separate from being gathered to his people.
Deut 32:50 And die on the mountain which you go up, and be gathered to your people, as Aaron your brother died in Mount Hor and was gathered to his people. Aaron died and was buried. Moses died and God buried him. In no sense is it possible to equate his being gathered to his people to simply being dead like they were. And, being in the wilderness, it is impossible that they were buried where their forebears had been.
Someday we shall “Gather at the River” with those Old Testament saints who’ve gone before, who, knowing less than we do, still walked by faith in hope of being together and with God.
Shall we gather at the river,
Where bright angel feet have trod
With its crystal tide forever
Flowing from the throne of God
Yes, we'll gather at the river,
The beautiful, the beautiful river,
Gather with the saints at the river,
That flows from the throne of God.