If you believe in the distinction between the gospel of the kingdom and the gospel of grace, I believe it will be the gospel of the kingdom that will be preached during the Tribulation.
Jesus stated that in Matthew 24
14 And
this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
15 When ye therefore shall see
the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand
16 Then let them which be in Judaea flee into the mountains:
17 Let him which is on the housetop not come down to take any thing out of his house:
18 Neither let him which is in the field return back to take his clothes."
19 And woe unto them that are with child, and to them that give suck in those days!
20 But pray ye that your flight be not in the winter, neither on the sabbath day:
21
For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
22 And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.