The "great multitude" starting at Rev.7:9 are shown having been saved (past tense) and standing before The Lamb at the end. Those represent the believing Gentiles that remain waiting on Christ Jesus and are thus 'sealed' also, though that chapter does not specifically say... they are sealed. No one is going to make it through the coming tribulation standing in Christ Jesus against the coming fake messiah without God's sealing. Rev.9 showed us that.
In that Rev.7:9 verse about the "great multitude", they are from all nations and peoples, too many to be numbered. That's why they are not given as a short list like those 144,000 of Israel are.
Also, I would warn any thinking that that "great multitude" do not go through the tribulation like the 144,000 do. The Rev.7 description about that multitude shows they came out of great tribualtion having washed their robes in the Blood of The Lamb (Jesus Christ). That definitely means they went through it, stood fast, and were saved at the end of it, and thus are gathered by Christ in final.