The Mass itself is the same. What changes is the musical accompaniment.
The differences are:
All the chants will be in Latin (even in the OF) except for the Kyrie
Instead of an entrance hymn, there will be an introit (entrance antiphon) for which the words are entirely scriptural (usually psalms)
At the penitential rite, the antiphon “Asperges me” or “Vidi aquam” (paschal time) will be used for blessing of holy water (if that rite is used), or after the penitential rite, the appropriate Kyrie (Kyrie Eleison, Christe Eleison, Kyrie Eleison) will be chanted in Greek
Instead of a responsorial psalm (except if using the Graduale Simplex) there will be a Gradual hymn, again taken from scriptural verses;
The Alleluia will include a scriptural verse chanted in Latin (replaced by a tract in Lent);
A Latin sequence may be chanted at some specific solemnities
One of the Gregorian credos will be chanted in Latin
There will be a scriptural offertory hymn chanted in Latin
Sanctus and Agnus will be chanted in Latin
There will be a communion antiphon, again scriptural, with psalm verse.
The chants will come from the official text, the Graduale Romanum (or for smaller churches with less able choirs, the Graduale Simplex), and will either be a cappella or with light organ accompaniment; the only musical instrument you’ll hear will be the organ either as accompaniment, or as a compliment to (i.e. an entrance piece played before the introit, an offertory piece after the hymn, an exit piece) the chants.
In short all the chants you will hear will draw its words from scripture. There will be no insipid or poorly composed, or even well-composed hymns. Non-scriptural hymns in Gregorian chant are for the most part limited to the Liturgy of the Hours. All of the chants you hear will be appropriate to the day (memorial, feast or solemnity), or season (Advent, Christmas, Lent, Paschal or OT), and to the cycle of readings in cases where chants vary with the A, B or C or Year I and Year II cycle of readings.
Finally, all of the above apply to the Ordinary Form Mass. The same will apply to the EF, but the chants themselves while from the same repertory, will not always be the same on any given day given the different liturgical year for the EF, though they will for the major solemnities and feasts.
