I wonder about something here... If the "little season" of Rev 20:3 is indeed 100 years long, then the following thought [from Great Controversy] raises a question.

"With military precision the serried ranks advance over the earth's broken and uneven surface to the City of God. By command of Jesus, the gates of the New Jerusalem are closed, and the armies of Satan surround the city and make ready for the onset." (GC 664)

Have the gates of the city been open up until this point?
It is clear that the ungodly cannot enter the city, and it is clear that there is no more probationary opportunity for them, but if the gates of the city are open for practically 100 years, why have they been open? Would the redeemed have any reason to be going out and in?

It is my understanding that the city is not resting upon terra firma at this point in time, but is still hovering over the earth until following the earth's cleansing by fire. The saints, however, can fly...and they have not been confined within the city as though it were a jail. Perhaps they were allowed to come out for a view of their former friends and relatives, or perhaps their liberty to come and go is a portion of the revelation which the wicked receive, showing what God is like.

I think the city's gates are shut only as a protection against the advance of the wicked at that point in time. Can we find other quotes about this?

The first thought that drew my attention, Green Cochoa, was that the city of God is "not resting upon terra firma at this point". Perhaps it is incidental in a way, but there does seem to be quite a lot of material saying that when Christ sets foot on the Mount of Olives, that mount will literally "part asunder", and a "great plain" will be formed between the two sections. The New Jerusalem physically descends and "rests" on the plain.

GC 663. "As the New Jerusalem, in its dazzling splendor, comes down out of heaven, it rests upon the place purified and made ready to receive it, and Christ, with His people and the angels, enters the Holy City. Now Satan prepares for a last mighty struggle for the supremacy."

And it appears that even in the lake of fire, the city is physically riding on the surface of the "boundless lake".

"...the whole earth, with the exception of that portion where the city rests, will be wrapped in a sea of liquid fire, yet the city is preserved as was the ark, by a miracle of Almighty power. It stands unharmed amid the devouring elements" (3SG 87, 7BC 986.5)

After the saints are changed to immortality, and are caught up together, with Jesus, receive their harps, crowns, &c., and enter the City, Jesus and the saints sit in judgment. The books are opened, the book of life and the book of death; the book of life contains the good deeds of the saints, and the book of death contains the evil deeds of the wicked. These books were compared with the Statute book, the Bible, and according to that they were judged. The saints in unison with Jesus pass their judgment upon the wicked dead. Behold ye! said the angel, the saints sit in judgment, in unison with Jesus, and mete out to each of the wicked, according to the deeds done in the body, and it is set off against their names, what they must receive at the execution of the judgment. This, I saw, was the work of the saints with Jesus, in the Holy City before it descends to the earth, through the 1000 years. Then at the close of the 1000 years, Jesus, and the angels, and all the saints with him, leaves the Holy City, and while he is descending to the earth with them, the wicked dead are raised, and then the very men that “pierced him,” being raised, will see him afar off in all his glory, the angels and saints with him, and will wail because of him. They will see the prints of the nails in his hands, and in his feet, and where they thrust the spear into his side. The prints of the nails and the spear will then be his glory. It is at the close of the 1000 years that Jesus stands upon the Mount of Olives, and the Mount parts asunder, and it becomes a mighty plain, and those who flee at that time are the wicked, that have just been raised. Then the Holy City comes down and settles on the plain. {ExV 33.2}

This is a relevant thought too, I think.

3SG 84.
"The city of God comes down and settles upon the mighty plain prepared for it. Then Jesus leaves the city surrounded by the redeemed host, and is escorted on his way by the angelic throng. In fearful majesty he calls forth the wicked dead. They are wakened from their long sleep. What a dreadful waking!"

Does the second resurrection take place while Jesus and the redeemed are OUTSIDE the city?