I know that only a priest or a deacon can read the gospel during mass but what if a lay person reads it does it make the mass not valid?
It makes the Mass not invalid, as there is no such thing as an “invalid Mass,” but it would be illicit.
Forgive my ignorance but would the Eucharist be valid?
Assuming there was a valid consecration, yes it would be. Few things make for an invalid consecration by a priest and they have nothing to do with who reads the Gospel.
Nothing that happens during the Liturgy of the Word can make the Eucharist invalid. It’s annoying but still a valid Eucharist.
There are only 4 things that make a mass invalid:
Lack of proper minister–validly ordained priest or bishop.
Lack of proper matter–pure wheat bread and pure grape wine with only a slight bit of water.
Lack of proper form–mangling the Words of Consecration such that they do not clearly refer to the Bread and Wine becoming the Body and Blood of Christ. This would require a deliberate act; accidentally stumbling over the words would not make the mass invalid.
Lack of intention to confect the Eucharist on the part of the celebrant. This is very hard to prove, btw. If the celebrant goes to the trouble of vesting, reading the prayers, and performing the directed ceremonial, intention is presumed.
Now, how does a layperson reading the Gospel enter into any of these?
Redemptionis Sacramentum makes it perfectly clear that the Gospel is to be read only by the deacon or the celebrant (if there is no deacon) and never by a lay person:
[63.] Within the celebration of the Sacred Liturgy, the reading of the Gospel, which is “the high point of the Liturgy of the Word”,139 is reserved by the Church’s tradition to an ordained minister.140 Thus it is not permitted for a layperson, even a religious, to proclaim the Gospel reading in the celebration of Holy Mass, nor in other cases in which the norms do not explicitly permit it.141
Now, the laity can assist in the proclamation of the Passion on Palm Sunday and Good Friday, since it is divided into three parts. But, the part of Christ is to be read by the celebant.
Actually, a lay person can read the Gospel under certain circumstances - I am thinking of the case of a priest friend of mine. He has macular degeneration, which makes it very difficult for him to read, so when there is no deacon available (and that is most of the time, since we have no permanent deacons yet), he will say "A reading from the Gospel according to…) and then a pre-selected lay person will do the actual reading.
Something like this is perfectly acceptable. When my friend reads the Opening Prayer, or any of the other prayers that he would not know by heart, he needs to pick the book up and hold it about 6 inches away from his face. Even then, he has difficulty reading the words.
I don’t think that this is allowed in the rubrics, as both the GIRM and RS make this very clear. In fact, Ecclesia de Mysterio also contains no such provision.
If the priest cannot see very well, how could he celebrate the rest of the Mass?
While I agree that the proclamation of the Gospel is reserved to the ordained, the priest celebrant is at the bottom of the pecking order, according to the GIRM:
59. By tradition, the function of proclaiming the readings is ministerial, not presidential. The readings, therefore, should be proclaimed by a lector, and the Gospel by a deacon or, in his absence, a priest other than the celebrant. If, however, a deacon or another priest is not present, the priest celebrant himself should read the Gospel…
That’s assuming that there even is either a deacon (1st choice) or another priest (2nd choice) present. If the celebrant is the only cleric at that Mass (which is the case for the vast majority of Masses celebrated), then there is no “pecking order” as he’s the only one who may proclaim the Gospel.