The elemental principles as observances of days, water, and fire
Gennadius is explaining what Paul meant by the elemental principles of the world, offering an alternative interpretation to the introductory law.
Or he calls the elemental principles days, water, and fire. The observances of these came from the law that led into slavery: days in sabbaths, new moons, and circumcisions; water in bodily purifications and baptisms; fire in not kindling fire on the sabbaths but eating stale food. But see what a strange thing Eusebius said about this in the fourth book of his Proof of the Gospel. "It is not for your good," Paul says, "that they welcome your same faith in Christ, but so that, having shut you out of it, they may persuade you to follow themselves. This has been contrived by them for your deception."
Greek
ἢ στοιχεῖα καλεῖ ἡμέρας, ὕδωρ, πῦρ ὧν ἦσαν ἐκ τοῦ εἰς δουλείαν ὑπαγαγόντος νόμου αἱ παρατηρήσεις· ἡμερῶν μὲν
ἔν τε σαββάτοις καὶ νουμηνίαις καὶ περιτομαῖς, ὕδατος δὲ ἐν ταῖς σωματικαῖς ἁγνείαις καὶ βαπτισμοῖς, πυρὸς δὲ ἐν τῷ μὴ ἀνακαίειν πῦρ ἐν τοῖς σαββάτοις ἀλλ' ἕωλα ἐσθίειν. ὅρα δὲ τί εἶπεν εἰς τοῦτο ξένον ὁ Εὐσέβιος ἐν τῷ τετάρτῳ λόγῳ τῆς Εὐαγγελικῆς ἀποδείξεως.{Gal 4,17 1} Οὐκ ἐπ' ἀγαθῷ, φησί, τὴν αὐτὴν ὑμῶν εἰς Χριστὸν πίστιν ἀσπάζονται, ἀλλ' ἵνα ὑμᾶς ταύτης ἐκκλείσαντες ἑαυτοῖς ἀναπείσωσιν ἕπεσθαι, πρὸς ὑμετέραν ἀπάτην τὸ τοιοῦτον αὐτοῖς μεμηχάνηται.