The Great Tribulation was in the first century AD in "this generation," just as Jesus prophesied.
The book of revelation talks about the great tribulation as well.
It's described by John in chapter 6.
Revelation 6
12 And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;
13 And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.
14 And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places.
15 And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains;
16 And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb:
17 For the great day of his wrath is come; and who shall be able to stand?
At this time of great tribulation John wrote that a great multitude which no man could number came out of it from all nations .
Revelation 7
9 After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
10 And cried with a loud voice, saying, Salvation to our God which sitteth upon the throne, and unto the Lamb.
11 And all the angels stood round about the throne, and about the elders and the four beasts, and fell before the throne on their faces, and worshipped God,
12 Saying, Amen: Blessing, and glory, and wisdom, and thanksgiving, and honour, and power, and might, be unto our God for ever and ever. Amen.
13 And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they?
14 And I said unto him, Sir, thou knowest. And he said to me,
These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.
Now you say the great tribulation took place in 70 Ad.How did this great multitude from all nations wash their clothes in the blood of the Lamb,before the gospel was preached to all nations?