The gathering is the gathering from heaven and earth. This is not the rapture of the Church.
Mark 13
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.
You of course LEFT OUT THE 24th & 25TH VERSES where Jesus gave the exact timing of His coming...
Mark 13:24-26
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.
KJV
So your pointing to the 6th SEAL is just you trying to use it as cannon fodder to prove your point, when it actually does not.
We get another view of this gathering here. It is the fall fruit harvest.
Rev 14
14 And I looked, and behold a white cloud, and upon the cloud one sat like unto the Son of man, having on his head a golden crown, and in his hand a sharp sickle.
15 And another angel came out of the temple, crying with a loud voice to him that sat on the cloud, Thrust in thy sickle, and reap: for the time is come for thee to reap; for the harvest of the earth is ripe.
16 And he that sat on the cloud thrust in his sickle on the earth; and the earth was reaped.
NONE of those above Scriptures give the actual TIMING of that symbolic harvest. Just because the word "harvest" is used doesn't mean you can latch whatever timing you want to it. Jesus showed in His command to learn a parable of the fig tree that 'summer' is the symbolic timing He comes to gather His faithful Church, not spring.
And here's a little Bible lesson your pre-trib rapture preachers won't tell you...
1. There's 2 Parts to the 6th Seal. The first part is about the coming Antichrist... in... the... spring...
Rev 6:12-13
12 And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;
13 And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.
KJV
Those stars falling to earth is directly... related to the result of the war in Heaven written in Revelation 12:7 forward. It's about Satan and his angels being cast down to earth into OUR earthly dimension, in plain sight, for the end of this world.
That
"untimely figs" idea is about real fig horticulture in the middle east. The EARLY fig, is one that grows in the winter, and FALLS OFF IN THE SPRING. As a spiritual metaphor for worship, it means BOWING IN WORSHIP TO THE WRONG MESSIAH, i.e., the first supernatural Messiah that comes to Jerusalem working great signs and wonders to deceive the world with. It means metaphorically an EARLY HARVEST time, which is NOT FOR CHRIST'S CHURCH. Instead, Jesus comes in the SUMMER HARVEST time, later to gather His Church.
2. The second part of the 6th Seal is about the actual "day of the Lord" when Jesus comes, and heaven departs as a scroll, and the mountains and islands moved out of their place.