Compare each phrase side by side.  But pay most attention to the highlights.

This is THE GOSPEL we are talking about. The main message of salvation…the MESSAGE we are to preach to the world! Let’s get it right!

This is Paul’s definition of the Gospel (or good news of Christ)

Paul tells us what was of FIRST importance! And the NWT changes that sentence to say this is just AMONG the first things handed on to them. (Just as Jws would say that These things about Jesus are only “among” the things they will preach to you in a study)

Look at the Greek interlinears. The first is from Bible hub. The second is The Kingdom Interlinear.  KIT adds a word that changes the meaning. See how they do this? Taking emphasis off from what Paul is clearly stating to be the “MAIN thing!”