The gathering together is detailed for us in 1Thessalonians. It is part and parcel with, if not, the same as, the "first resurrection".
"13Brothers, we do not want you to be ignorant about those who fall asleep, or to grieve like the rest of men, who have no hope. 14We believe that Jesus died and rose again and so we believe that God will bring with Jesus those who have fallen asleep in him. 15According to the Lord's own word, we tell you that we who are still alive, who are left till the coming of the Lord, will certainly not precede those who have fallen asleep. 16For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. 17After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever." - 1Thessalonians 4:13-17 NIV
The resurrection of the righteous and the unrighteous is a separate, singular event. Both events, the "first resurrection" and the "resurrection of the righteous and the unrighteous" are described in Revelation.
"4I saw thrones on which were seated those who had been given authority to judge. And I saw the souls of those who had been beheaded because of their testimony for Jesus and because of the word of God. They had not worshiped the beast or his image and had not received his mark on their foreheads or their hands. They came to life and reigned with Christ a thousand years. 5(The rest of the dead did not come to life until the thousand years were ended.) This is the first resurrection. 6Blessed and holy are those who have part in the first resurrection. The second death has no power over them, but they will be priests of God and of Christ and will reign with him for a thousand years." - Revelation 20:4-6
The term "resurrection of the just and the unjust" denotes the far-reaching nature of the resurrection. It is not a teaching of two separate resurrections, one for "the just", and one for "the unjust". But rather it is alluding to the propitiatory nature of the sacrifice of Christ and the restorative benefits of the Ransom. Paul spoke of this in his first letter to Timothy.
"3This is good, and pleases God our Savior, 4who wants all men to be saved and to come to a knowledge of the truth. 5For there is one God and one mediator between God and men, the man Christ Jesus, 6who gave himself as a ransom for all men—the testimony given in its proper time." - 1Timothy 2:3-6
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