To answer your question, I believe the 144k represent the Jewish converts to Christianity and the other Sheep talked about in the gospels are the gentiles. Both groups are gathered in heaven before the throne of god as stated at chapters 7,14,19 of Revelations. These also join to together r with resurrected ones mentioned at chapter 5 and in verses 9-10 are said to reign on earth as priests . This is in reference to an older scripture I believe in Deuteronomy which says something to the effect of, the nation of Israel is a holy nation of royal priests’ .
The JWs I believe to make for themselves a laity class made the 144k a special group that go to heaven to rule as kings over the great crowd now residing on earth. Of course the leaders are always of the 144k . This can easily be proven wrong because again the scriptures at Revelations 5:9-10 in the Greek clearly state they will not rule and are not kings and are on the earth.
It can also be shown that the great crowd is in heaven using the Bible but again the JWs change the meaning of the Greek word Naos so they can claim the crowd is in the earth.