Specifically, the Son of Man will send angels to assemble the saints from everywhere in heaven and earth. The bifurcation in the description is important: one aspect of the gathering is earthly, the other aspect heavenly. The expression, "from the four winds" refers to the four compass directions and entails every place on earth; "from one end of heaven to the other" refers to the realms beyond the skies and is heavenly. We would have to assume that at least the dead in Christ were part of the heavenly group, but there is no textual reason why it wouldn't or couldn't include the already raptured.
Those gathering angels are sent with a trumpet call, which might lead the inquiring reader to ask, "Exactly which trumpet blast might that be?" A succession of seven trumpet calls are mentioned in Revelation but none of them are associated with the gathering of the saints. I do not believe they are what is referenced by Christ in the Olivet Discourse. A last trump is mentioned in 1 Corinthians 15:52, and is definitely associated with the Rapture, but I don't believe that it is associated with the seven trumpets of Revelation either.
The seventh trumpet of the Apocalypse leads to more tribulation (i.e, the seven bowls) rather than the Return of Christ after tribulation is finished. The seven trumpets are not referenced anywhere else in scripture (even though modeled in a sense at Jericho), and I do not think the Apostle Paul would have even known of them when writing to the Corinthians in 55 CE (at the latest). John wrote the Apocalypse in 95 CE, and I doubt the Apostle Paul had the same type of revelation or awareness of detail as John would have prior to him receiving his vision on Patmos. There is just no good reason to associate the last trump of 1 Corinthians with the seven visionary trumpets of Revelation.
Last (Koine: esxate, meaning final, extreme) certainly implies more than one, but if it's not referring to the series of seven in the Apocalypse, then what is it referring to? Perhaps Israel's mandate to use trumpets during the exodus can provide some insight.
A long trumpet blast, not followed by another, in other words, the last signal trump sounded meant all Israel was to gather. It seems reasonable to me that Paul could have had this in mind when writing to the Corinthians (or even that a succession trumpet blasts were signaled when they set out by camp, the last signaling that all Israel was on the move). If so, then Paul was merely relying upon that imagery in conjunction with the word "last" to get across the sense of totality and finality in God's people moving into their eternal state, rather than connecting this to any series of trumpet blasts.
So, at the end of this age, immediately after the Great Tribulation, there will be an actual trumpet blast, unconnected to the seven visionary blasts of the Apocalypse, which signals the final ingathering of the saints in heaven and on earth to be with Christ. All the faithful dead will have been raised at that time, and those alive and remaining will have been changed. That doesn't mean that some of either category won't have been raised or changed before, only that all those that will be will have been so at that particular time.