"This gospel of the kingdom shall be preached in the whole world as a testimony to all the nations, and then the end will come." Matthew 24:14
The Online Study Bible makes it easy to see that 16 out of 18 translations use the word "This" in Matthew 24:14. One of the two that do not use this uses "these" while the only noncommital version simply says, "And the Good News about the Kingdom will be preached throughout the whole world,"
Typically most of us erroneously think like the New Living Translation states it: "And the Good News about the Kingdom will be preached throughout the whole world, so that all nations will hear it; and then the end will come." with the word "the" instead of "this" implying all the gospel as considered to be the great commision:
And He said to them, "Go into all the world and preach the gospel to all creation." Mark 16:15
"The gospel" carries the connotation of the whole gospel, while "this gospel" strongly suggests that part which was said in close proximity of the word this being said.
Now with this insight on the significance of the word this used in Matthew 24:14 let's read from the beginning of Matthew 24 through verse 14 and consider what kind of meaning the intentional use of the word this has!