The priest is not permitted to change the words of the Mass. The gospel is part of the Mass. (Now, there are times when alternate gospels may be used, but they still must be approved alternates. Also, most churches use the NAB Bible readings but some use other Bible translations).
So for the priest to make a “running commentary” on the gospel is rather like the equally illicit practice of some priests of making a “running commentary” on the Mass, DURING the Mass.
It is wrong. It renders the Mass illicit (but not invalid). Meaning, it is a wrong thing to do, but the Mass itself, while it has a wrong thing in it, is still okay regarding its main purpose.
If the priest starts making a running commentary on the words of the consecration, THAT would probably make the Mass invalid–meaning that there would BE no consecration. The purpose of the Mass would not be fulfilled. Receiving at “communion” one would only receive bread and wine, not the Body and Blood of Christ.