The Great Multitude are the 144,000
The reason is that first John HEARS the number of God’s servants sealed (he does not SEE the number of those who are sealed at this point just hears):
“Do not harm the earth, the sea, or the trees till we have sealed the SERVANTS of our God on their foreheads.” And I HEARD the number of those who were sealed. One hundred and forty-four thousand of all the tribes of the children of Israel were sealed"
So at this point John HEARS that God’s servants are 144,000 from all the tribes of Israel. He doesn’t SEE anything yet. But what does he SEE with his eyes just after hearing this?
“After these things I LOOKED, and behold, a great multitude which no one could number, of all nations, tribes, peoples, and tongues, standing before the throne and before the Lamb, clothed with white robes”
What John is LOOKING at are 144,000 people in white robes, 2 super bowl football stadiums of people in white robes from all “nations, tribes, peoples, and tongues". Now in the eyes of John… that is a great multitude “which no one could number”. And of course all the “nations, tribes, peoples, and tongues” are the 12 tribes of Israel because they are all the seed of Abraham in Christ and they are all the SERVANTS of God who get sealed.
There will be no ethnic divisions in the Kingdom of God where you have 144,000 Jews who can sing a song nobody else can and then you have these other racially different groups who can’t sing that same song (Rev 14:3).