In his work De hominis opificio (On the making of man), in the praefatio, Gregory alludes explicitly to a list of chapter titles for the work:
…and for clearness’ sake I think it well to set forth to you the discourse by chapters, that you may be able briefly to know the force of the several arguments of the whole work.
The Greek text of Migne does not print any chapter titles, but the English translators embedded them following this sentence.
The word rendered “chapters” is, of course, kephalaia. Here at least we see that Gregory is familiar with the idea of a work which may be summarised in this way.
-  PG 44, col. 128B. English translation from the Nicene and Post-Nicene Fathers series, which then inserts a translation of the chapter headings which do not appear at this position in the Greek text of Migne. ↩