“And he shall send his angels with a loud trumpet call and they will gather his elect from the four winds, from one end of the heavens to the other.” ( Mt 24 : 31 )
You might have noticed that in this passage from Matthew, he only mentions about the gathering of the elect who are dead, from the ends of the heavens and not the gathering of the elect who are living, from the ends of the earth, probably because none of the elect survived the tribulation. The only ones who will be protected during the tribulation and escape from being beheaded are the 144,000 from the tribes of Israel ( Rev 7 : 4 ) because they had the seal of the living God on their foreheads ( Rev 7 : 3 ). They are the first fruits from among the Jews and they will be ‘snatched’ (Greek : Harpazo, the same word used in 1 Thess 4 : 17 to denote Rapture ) upto God (Rev 12 : 5 ) and will be seen with the Lamb ( Rev 14 : 1-3 )
As said earlier, the gathering of the saints who are dead in Christ and those who are still alive and left, at his secret coming in the clouds, will happen at Rapture and they will be with the Lord forever ( 1 Thess 4 : 17). So they are not the ones who need to be gathered again at the second coming of Jesus because they will have been with him during the tribulation and they will be also be with him at his second coming ( Rev 19 : 14, Jude Vs 14 ) But it is the tribulation saints ( both Gentiles and Jews ) who will be beheaded during the tribulation, who needs to be gathered ( not in the heavens but on earth because it will happen after the second coming of Jesus, when he takes his seat on the throne in Jerusalem – Lk 1 : 32, Mt 25 : 31 )
Here then are the references about the elect who will be beheaded :-
‘I saw under the altar souls of those ( mostly Gentiles ) who had been slain because of the word of God and the testimony they had maintained.’ ( Rev 6 : 9 )
‘And I saw … those ( mostly Jews ) who had been victorious over the beast and his image and over the number of his name. They held harps .. and sang the song of Moses …’ ( Rev 15 : 2 – 3 )
Here is the reference of these two groups being gathered together :-
‘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 worshipped 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. The rest of the dead ( the wicked in Hades ) did not come to life until the thousand years were ended.’ ( Rev 20 : 4 )
( The wicked people who survive the tribulation, will go into the millennium but from among them and their children, many will be gathered by Satan , at the end of the thousand years, for the final rebellion against God – Rev 20 : 8 – 9)
So coming back to your question: Will all the elect be slain? It is not explicitly said so in the Scriptures but it is implicit .
‘Then I heard a voice from heaven say,” Blessed are the dead who die in the Lord from now on.” ( Rev 14 : 13 )
But if some of the elect escape from being beheaded and survive the tribulation, they will have to be gathered ( not in the heavens but on earh because it will happen after the second coming of Jesus, when he takes his seat on the throne in Jerusalem ), before the Millennial rule of Christ, provided they never worshipped the image or took the mark of the beast (Rev 14 : 9 – 11 ). Probably that could be the reason why Mark also added : ‘from the ends of the earth …’ ( Mk 13 : 27 ). But that reference doesn’t alter the course of events nor support the arguement for a post-trib rapture.