so the Church are allowed to change the Gospel text in the Liturgy?
It not a matter of changing the text. Remember that the Gospels were written in Greek not Latin so if you are reading it in any language other that Greek it is a translation. And Yes translations do change. and the one doing the translating is allowed to determine how that translation is done. Here is a good example. the red is mine.
John 21 15When therefore they had dined, Jesus saith to Simon Peter: Simon son of John, lovest thou me more than these? He saith to him: Yea, Lord, thou knowest that I love thee. He saith to him: Feed my lambs. 16He saith to him again: Simon, son of John, lovest thou me? He saith to him: Yea, Lord, thou knowest that I love thee. He saith to him: Feed my lambs. 17He said to him the third time: Simon, son of John, lovest thou me? Peter was grieved, because he had said to him the third time: Lovest thou me? And he said to him: Lord, thou knowest all things: thou knowest that I love thee. He said to him: Feed my sheep.
In English it never translated this way but it could be.
15So when they had finished breakfast, Jesus said to Simon Peter, “Simon, son of John, do you have an unconditional love for Me greater more than these?” He said to Him, “Yes, Lord; You know that I am your best friend.” He said to him, “Tend My lambs.” 16He said to him again a second time, “Simon, son of John, do you do you have an unconditional love for Me?” He said to Him, “Yes, Lord; You know that I am your best friend.” He said to him, “Shepherd My sheep.” 17He said to him the third time, “Simon, son of John, am I your best friend?” Peter was grieved because He said to him the third time, And he said to Him, “Lord, You know all things; You know that I love unconditionally.” Jesus said to him, “Tend My sheep.