Mat 16:18 AndG1161 I say alsoG2504 G3004 unto thee,G4671 ThatG3754 thouG4771 artG1488 Peter,G4074 andG2532 uponG1909 thisG5026 rockG4073 I will buildG3618 myG3450 church;G1577 andG2532 the gatesG4439 of hellG86 shall notG3756 prevail againstG2729 it.G846
From Vines Complete Expository Dictionary of the Bible
Gates
1. pule (G4439) is used (a) literally, for a larger sort of "gate," in the wall either of a city or palace or temple, Luk_7:12, of Nain (burying places were outside the "gates" of cities); Act_3:10; Act_9:24; Act_12:10; Heb_13:12; (b) metaphorically, of the "gates" at the entrances of the ways leading to life and to destruction, Mat_7:13, Mat_7:14; some mss. have pule, for thura, "a door," in Luk_13:24 (see the RV); of the "gates" of Hades, Mat_16:18, than which nothing was regarded as stronger. The importance and strength of "gates" made them viewed as synonymous with power. By metonymy, the "gates" stood for those who held government and administered justice there.
Hades
hades (G86), "the region of departed spirits of the lost" (but including the blessed dead in periods preceding the ascension of Christ). It has been thought by some that the word etymologically meant "the unseen" (from a, negative, and eido, "to see"), but this derivation is questionable; a more probable derivation is from hado, signifying "all-receiving." It corresponds to "Sheol" in the OT. In the KJV of the OT and NT; it has been unhappily rendered "hell," e.g., Psa_16:10; or "the grave," e.g., Gen_37:35; or "the pit," Num_16:30, Num_16:33; in the NT the revisers have always used the rendering "hades"; in the OT, they have not been uniform in the translation, e.g. in Isa_14:15 "hell" (marg., "Sheol"); usually they have "Sheol" in the text and "the grave" in the margin. It never denotes the grave, nor is it the permanent region of the lost; in point of time it is, for such, intermediate between decease and the doom of Gehenna. For the condition, see Luk_16:23-31.
The word is used four times in the Gospels, and always by the Lord, Mat_11:23; Mat_16:18; Luk_10:15; Luk_16:23; it is used with reference to the soul of Christ, Act_2:27, Act_2:31; Christ declares that He has the keys of it, Rev_1:18; in Rev_6:8 it is personified, with the signification of the temporary destiny of the doomed; it is to give up those who are therein, Rev_20:13, and is to be cast into the lake of fire, Rev_20:14.
Note: In 1Co_15:55 the most authentic mss. have thanatos, "death," in the 2nd part of the verse, instead of "hades," which the KJV wrongly renders "grave" ("hell," in the marg.).