What gospel will the 144,000 preach during the tribulation?
The sealed 144,000 will preach the gospel of the Kingdom during the tribulation, not the gospel of grace of the church age. Their message is the basic one John the Baptist instituted and Christ and the Twelve preached: the King is coming, which is good news for that period.
Les explains that these sealed witnesses do not carry the gospel of grace that has been preached during the church age. Instead, they go back to the very basic message John the Baptist instituted and that Christ and the Twelve preached: the gospel of the Kingdom. That message means the King is coming, and it is good news for that period of history. This takes place as they are sealed and sent out to encircle the globe, carrying a kingdom proclamation rather than the church-age gospel of grace.