Have you studied 1 Thessalonians 4:15-17 thoroughly? I know pre-trib likes to pull from that chapter a lot, but I find many on pre-trib haven't properly understood it.
Matt 24:29-31
29 Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
30 And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory.
31 And He shall send His angels with a great sound of a trumpet, and they shall gather together His elect from the four winds, from one end of heaven to the other.
KJV
That specific gathering is about the asleep saints Jesus brings with Him on the day of His coming:
1 Thess 4:13-14
13 But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope.
14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with Him.
KJV
That is the specific link to that gathered group in the Matt.24:31 verse. They are gathered in their resurrected bodies, and Paul said the asleep saints resurrect first.
Mark 13:24-27
24 But in those days, after that tribulation, the sun shall be darkened, and the moon shall not give her light,
25 And the stars of heaven shall fall, and the powers that are in heaven shall be shaken.
26 And then shall they see the Son of man coming in the clouds with great power and glory.
27 And then shall He send His angels, and shall gather together His elect from the four winds, from the uttermost part of the earth to the uttermost part of heaven.
KJV
The Mark 13 example of that gathering is different. It instead is about those still alive on earth being gathered:
1 Thess 4:17
17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
KJV
1 Thessalonians 4:15-16 tells us those of us still alive on earth will not precede (prevent = beforehand in the Greek) the asleep saints which will resurrect first.
It's also important to understand that event accompanies the 'change' to the "spiritual body" which Paul taught in 1 Corinthians 15, what Paul called the "image of the heavenly". The word "air" in 1 Thess.4:17 can also mean the 'breath' per the Greek 'aer' (Strong's no.109), pointing to the breath of life spirit body. Basically it mean, on that day, the heavenly dimension is going to be opened up to all peoples. This present existence in a flesh body will be over. That's why Rev.20 doesn't mention a flesh death anymore, but only the "second death" which is the casting into the future "lake of fire", as it was prepared especially for the devil and his angels, and they never had flesh bodies.