The “gentiles” are not an afterthought. That would make the Plan of Salvation something the Lord makes up as He goes, and we know from the bible that the Plan was in effect before Creation. Christ’s sacrifice on the cross is not an after thought. God knows everything that ever was and ever will be. There are no after thoughts, not even the least living soul anywhere. The whole idea of it is an absurdity in theology.
Then show me where Jesus preaches to the Gentiles. If anything He says the opposite. The so-called "Great Commission" (Math 28) is most likely an addition. We have no originals, so we have to take the 4th century bibles we have with a grain of salt and parse what is uncharacteristic of the rest of the writing, and the Great Commission just doesn't fit the bill.
Or, conversely, since Matthew's Gospel is written in the late 70's of the 1st c. AD, it was a latter-day revelation for Matthew that this is what God wanted them to (Great Commission), so he added it.
You must understand that in those days when someone quoted a figure it was what the author thought the figure would have said and not necessarily what the figure actually did say.
Additions to Gospels were made, that much is known. Someone added a dozen or so verses to Mark's last chapter. Comma Johannenum {1 John 5:7-8) is an addition, as well. So is the so-called Pericope Adulterae (John 7:53-8:11), etc.
Reading the Bible is not knowing the Bible.