Correct, the 144,000 are part of the Great Multitude of the Christian peoples, who have stood firm in their faith during the recently happened Sixth Seal devastation.
They are all in the holy Land and Jesus reveals Himself to them, 1 Thess 1:10; as He selects 12,000 out from each of the twelve groups they will be divided into. Revelation 14:1-5
Keras, first, Jesus promised all believers in Rev 3:10 that he would keep us from the Trib. That means all believers who are alive immediately before the start of the Trib, will be kept from entering it. 1 Th 1:10 is about how Jesus "delivers us" ("snatches us") up, up and away from Earth and straight into Heaven through 1 Th 4:16-17, 1 Cor 15:52 and Rev 4:1. You can't prove apostle John ever left the island of Patmos during his vision of Revelation. Therefore, Rev 4:1 is entirely a future event, known as the pre-Trib rapture of the Church, straight to Heaven.
You are misinterpreting Rev 7. The 144k are not part of the GM. Rev 7:1-8 say (in verse 3) that the 144k are servants of God who come from 12 tribes (verses 5-8). Rev 7:1-8 are all about the 144,000 sealed servants of God from 12 tribes.
The second half of Rev 7 (verses 9-17) are exclusively about the GM, who are described in verse 9 as "a great multitude that no one could number..."). The 144k are never said to be, in your words, "part of the Great Multitude of the Christian peoples..."). You are making that up.
The GM is composed of new converts in the Trib who are martyred. The new converts come about because many unbelievers in the Trib come to their senses, repent and accept Jesus Christ as their Savior. They are killed for their testimony of Jesus Christ (Rev 6:9). The martyrs from the GT are found in Rev 12:17 and Rev 13:7 & 15. They are killed for keeping the commandments of God and for holding to the testimony of Jesus (Rev 12:17) or for refusing to worship the image of the beast (Rev 13:15). They become the "fellow servants" cited in Rev 6:11. The martyrs from the GT also end up under the altar.
All of the martyrs are resurrected in the last sentence of Rev 20:4. They are the GM. They are millions of new converts who came to Christ in the Trib, and who were martyred for their testimony of Jesus Christ or for refusing to worship the beast. After their resurrection in Rev 20:4, the GM is seen in Heaven, in Rev 7:9.
The 144k do not go to Heaven until Rev 14:3. That further proves that the 144k are not part of the GM. The 144k are instrumental in causing the greatest revival in the Trib, by leading a multitude to Christ.