The elemental principles as guardians, not sun and moon
Gennadius is explaining Galatians 4:3, arguing that Paul's 'elemental principles of the world' are not the heavenly bodies but the introductory law given to the Jews.
Paul took the elemental principles of the world in the rank of guardians and stewards who tutor the immature in age toward what is beneficial—principles to which we were, because of our immature knowledge, beneficially enslaved (Galatians 4:3). He would not, I think, have taken sun and moon into the rank of guardians, because subjection to them would be unprofitable. Rather, by the elemental principles of the world he means either the elementary introductory law—for he was writing to believers from among the Jews, and both what he said a little earlier and the statement “if you are circumcised, Christ will benefit you nothing” (Galatians 5:2) make this clear.
Greek
4 {Gal 4,3} Ἐν τάξει τῶν ἐπιτρόπων καὶ οἰκονόμων τῶν τοὺς ἀτελεῖς τῇ
ἡλικίᾳ πρὸς τὸ συμφέρον παιδαγωγούντων ἔλαβε τὰ στοιχεῖα τοῦ κόσμου οἷς ἦμεν
διὰ τὸ τῆς γνώσεως ἀτελὲς συμφερόντως δεδουλωμένοι. οὐκ ἄν, ὡς οἶμαι, ἥλιον καὶ
σελήνην ἔλαβεν εἰς τάξιν τῶν ἐπιτρόπων διὰ τὸ ἀσύμφορον εἶναι τὴν εἰς ταῦτα
ὑποταγήν, ἀλλὰ στοιχεῖά φησι τοῦ κόσμου ἢ τὸν στοιχειώδη εἰσαγωγικὸν νόμον· καὶ
γὰρ τοῖς ἐξ Ἰουδαίων πιστοῖς ἔγραφεν, καὶ δηλοῖ τά τε μικρὸν ἀνωτέρω εἰρημένα καὶ
τὸ ἐὰν περιτέμνησθε Χριστὸς ὑμᾶς οὐδὲν ὠφελήσει.