encyclopedia.com states
it is a Jewish expression referring the death
"The phrase "the gates of Hades–Sheol" occurs in the Old Testament at Is 38.10 and Wis 16.13,
where it is a figurative expression for death. This is likewise its meaning in the apocrypha (Psalms of Solomon 16.2; 3 Mc5.51)
and in classical Greek literature (Homer, Iliad 5.646; 9.312; Odyssey 14.156; Aeschylus, Agamemnon 1290; Euripides, Hippolytus, 56)
GATES OF HELL This phrase occurs in the New Testament only at Mt 16.18: "… upon this rock I will build my Church, and the gates of hell shall not prevail against it." The Greek word here rendered "hell" is
www.encyclopedia.com
From Gotquestions
"It is clear that
Jesus was declaring that death has no power to hold God’s people captive. Its gates are not strong enough to overpower and keep imprisoned the church of God."
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From US Conference of Catholics Bishops states?
"The gates of the netherworld shall not prevail against it: the netherworld (Greek
Hadēs, the abode of the dead) is conceived of as a walled city whose gates will not close in upon the church of Jesus, i.e.,
it will not be overcome by the power of death."