As he mentions in the stele of Nicopolis ad Istrum, he was an assistant stonecutter, son of Proteos.
The Tauroctony of Nicopolis ad Istrum is unique as it is the only Mithraic stele befitting a Greek donor.
Ἀγαθῆι τύχηι. Ἡλίῳ | Μίθρᾳ | θεῷ | ἐπηκόῳ || Αὐρ(ήλιος) | Μᾶρκος | γναφεὺς | τὸ συήλιον | σὺν τῇ || ζωγραφίᾳ | κατεσκεύ|ασεν | ἐκ τῶν |ἰδίων || εὐχαρισ|τήριον.
To the lord Mithras, as an ex-voto, Galerios, son of Proteos, assistant stone cutter. To good fortune. To Helios Mithras, the listening god, Aurelios Markos, stone cutter, had the stele erected with the painting, at his own expense, as a mark of gratitude.