The interpretation timeline
Revelation 21:21 Douay-Rheims

And the twelve gates are twelve pearls, one to each: and every several gate was of one several pearl. And the street of the city was pure gold, as it were transparent glass.

A.D. 303 – 1871 5 witnesses

Patristic

before A.D. 750
303
A.D.
c. A.D. 250–303
“The city which he says is squared, he says also is resplendent with gold and precious stones, and has a sacred street, and a river through the midst of it, and the tree of life on either side, bearing twelve manner of fruits throughout the twelve months; and that the light of the sun is not there, because the Lamb is the light of it; and that its gates were of single pearls; and that there were three gates on each of the four sides, and that they could not be shut. I say, in respect of the square city, he shows forth the united multitude of the saints, in whom the faith could by no means waver. As Noah is commanded to make the ark of squared beams, that it might resist the force of the deluge, by the precious stones he sets forth the holy men who cannot waver in persecution, who could not be moved either by the tempest of persecutors, or be dissolved from the true faith by the force of the rain, because they are associated of pure gold, of whom the city of the great King is adorned. Moreover, the streets set forth their hearts purified from all uncleanness, transparent with glowing light, that the Lord may justly walk up and down in them.”
Source

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“And I saw no temple therein,.... No material temple, as was in the old Jerusalem, or such as is described in Ezekiel's vision; nor any place of public worship, as under the Gospel dispensation; for in this state there will be no such external form of worship as now, such as preaching the word and administering ordinances. The Jews expect a third temple, but in vain. For the Lord God Almighty, and the Lamb, are the temple of it; God will be immediately present with his people, whose face they shall see, and whom they will serve in the most pure and spiritual manner; and Christ in his human nature, in the temple of his body, that tabernacle which God pitched, and not man, which is filled with the train of the divine perfections, and in which the fulness of the Godhead dwells bodily, will be the only medium of the divine Presence, and of the communications of glory to men, and of the saints' praise to God, which will be the service they will be employed in; and the Lamb being joined with the Lord God Almighty, shows his deity and his equality with his Father.”
Source
1832
A.D.
Adam Clarke Methodist
1762–1832
“The twelve gates were twelve pearls - This must be merely figurative, for it is out of all the order of nature to produce a pearl large enough to make a gate to such an immense city. But St. John may refer to some relations of this nature among his countrymen, who talk much of most prodigious pearls. I shall give an example: "When Rabbi Juchanan (John) once taught that God would provide jewels and pearls, thirty cubits every way, ten of which should exceed in height twenty cubits, and would place them in the gates of Jerusalem, according to what is said Isa 54:12, I will make thy windows of agates, and thy gates of carbuncles, one of his disciples ridiculed him, saying, Where can such be found, since at present there is none so large as a pigeon's egg? Afterwards, being at sea in a ship, he saw the ministering angels cutting gems and pearls; and he asked them for what purpose they were preparing those. They answered, to place them in the gates of Jerusalem. On his return he found Rabbi Juchanan teaching as usual; to whom he said, Explain, master, what I have seen. He answered, Thou knave, unless thou hadst seen, thou wouldst not have believed; wilt thou not receive the saying of the wise men? At that moment he fixed his eyes upon him, and he was reduced into a heap of bones." - Bava bathra, fol. 77, 1, and Sanhedrim, fol. 100, 1, page 393. Edit. Cocceii. See Schoettgen.”
Source
1871
A.D.

Undated

date unknown
date
unknown
Glossa Ordinaria Medieval
“And twelve gates. As if he said: I have said that there are twelve gates, and since those twelve gates are so, that is, the prophets leading in to the faith of the Trinity out of the four parts of the world. Through each one, that is, through the instruction of individuals; they are pearls, that is, their imitators, who, from whatever part of the world they be called to the faith of the Trinity, are splendid in virtues. Each of the gates, etc. As if he said: In order that they might be gates, it was necessary that there be pearls. The streets of the city gold. That is, full of wisdom, or of brightness, clean from criminal stain. Glass, transparent. That is, hiding no filth in themselves. Or because the wicked can recognize their own wickedness in the works, as in a mirror.”
Source
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.