It can be argued that there can be an introduction before the Gospel and a homily after it.
The argument for the introduction, is from the 1981 Introduction to the Lectionary for Mass:
“15. There may be concise introductions before the readings, especially the first. The style proper to such comments must be respected, that is, they must be simple, faithful to the text, brief, well prepared, and properly varied to suit the text they introduce.”
Pope Benedict in the 2007 Sacramentum Caritatis:
“**The liturgy of the word **
45. Together with the Synod, I ask that the liturgy of the word always be carefully prepared and celebrated. Consequently I urge that every effort be made to ensure that the liturgical proclamation of the word of God is entrusted to well- prepared readers. Let us never forget that “when the Sacred Scriptures are read in the Church, God himself speaks to his people, and Christ, present in his own word, proclaims the Gospel”(135). When circumstances so suggest, a few brief words of introduction could be offered in order to focus the attention of the faithful. …”
The 2002 GIRM also allows introductions “before the readings”:
“31. … In addition, he may give the faithful a very brief introduction to the Mass of the day (after the initial Greeting and before the Act of Penitence), to the Liturgy of the Word (before the readings), and to the Eucharistic Prayer (before the Preface),”
Does “before the readings” mean before all the the readings, or before each reading? I think it could be understood as before each reading, as the 1981 Introduction to the Lectionary clearly allows.
But if the priest blends the two, so people are confused about what it the Gospel and what is the introduction, he has broken rules.
From the Introduction to the Lectionary for Mass:
“12. In the celebration of Mass the biblical readings with their accompanying scriptural chants may not be omitted, shortened, or, worse still, replaced by nonbiblical readings.”
From the 2002 GIRM:
“309. … From the ambo only the readings, the responsorial Psalm, and the Easter Proclamation (Exsultet) are to be proclaimed; it may be used also for giving the homily and for announcing the intentions of the Prayer of the Faithful. The dignity of the ambo requires that only a minister of the word should go up to it.”
So the Gospel must be from the ambo. The homily may be from the ambo. The words of introduction to the Gospel should never be from the ambo.