Adam Clarke on Numbers 31:2

Gathered unto thy people - Where? Not in the grave surely. Moses was gathered with none of them, his burial-place no man ever knew. "But being gathered unto one's people means dying." It does imply dying, but it does not mean this only. The truth is, God considers all those who are dead to men in a state of conscious existence in another world. Therefore he calls himself the God of Abraham, and of Isaac, and of Jacob; now God is not the God of the dead, but of the living; because all Live to Him, whether dead to men or not. Moses therefore was to be gathered to his people - to enter into that republic of Israel which, having died in the faith, fear, and love of God, were now living in a state of conscious blessedness beyond the confines of the grave. See the note on Gen 25:8, and Gen 49:33 (note).