When a church enters communion with Rome, as did part of the Armenian church, the Holy See accepts as valid all of its spiritual and theological patrimony, including canonizations, so long as there is no explicit contradiction with Catholic doctrine.
This is what happened when the Armenian Catholic Church was formed, regarding St. Gregory and his work. Also, it appears, St. Gregory was persecuted in life for defending the Council of Chalcedon, which would also count in his favour.
The Definition of Chalcedon:We, then, following the holy Fathers, all with one consent, teach people to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten God (μονογενῆ Θεὸν), the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.