From this record he derived considerable aid in arguing with the Eutychians during their ascendancy under the usurper Basiliscus, A. D. 475-477 ; and this induced him to collect further information respecting the Council, from Joannes, Eusebius of Caesareia, Rufinus, and others. He embodied the information thus collected in a work termed by Photius Πρακτικὸν τῆς Πρώτης Συνόδου ἐν τρισί τόμοις; ; but, as Photius remarks, it is as much entitled to the name of History as of Acts.

The work is extant in the different editions of the but it has been suspected that the third part, or book, has been mutilated or corrupted by the earliest editors, in order to get rid of the testimony which (judging from the abstract of Photius) it afforded, that Constantine was not baptized at Rome by Pope Sylvester.

The first book comprehends the history of Constantine to his victory over Licinius. The second comprehends the history of the Council; and contains some discussions between certain " philosophers," advocates of " the impious Arius and the blasphemies invented by him," and the " holy bishops" of the opposite party; which discussions Cave believes to be pure inventions either of Gelasius or of the author of the ancient manuscript which formed the basis of his work. The third book, as we now have it, contains only a few letters of the emperor Constantine.

Baronius ascribes to Gelasius of Cyzicus a treatise against the Eutychians and Nestorians, of which he supposes the work , which is commonly regarded as the original Latin work, and passes under the name of Pope Gelasius I., to be only a version. Baronius does not appear to have many supporters in this supposition. It may be observed that one manuscript used by Photius of the was anonymous, but in another the work was inscribed " By Gelasius, bishop of Caesareia in Palestine." This inscription probably originated in a mistake. Photius could not find out who the author of the work was further than he had described himself in the preface, but says that there had been two, if not three, bishops of Caesareia of the name.