Hi Sopho,
This quote was attributed to Gregory of Nyssa by John Wesley Hanson in his book: Aion-aionios - an Excursus on the Greek Work Rendered Everlasting, Eternal, Ect., in the holy Bible.
(see archive.org/stream/aionaioniosexcur00hans#page/154/mode/2up/search/Whoever).
John Hanson says that Gregory stated this in his treatise: Concerning Infants Who Have Died Prematurely; but I failed to find the quote.
(see sage.edu/faculty/salomd/nyssa/infants.html).
My guess is that Gregory did not make this quote, or, if he did, it is found in a completely different work. My electronic version of Gregory’s writings found no such quote, but that does not prove he did not write this; there are several different translations of Gregory that use different words not found in a search strings.
In short, I can rule out that Gregory’s book, De Infantibus, does not contain this quote, or even words to the effect. John Hanson, I suggest, made a mistake…, and everyone who quotes this from J.W. Hanson follows the same mistake.
Steve