A 410 Discussion : Contrary to a Legendary Action that would say "cast a spell that takes 1 action to cast" (for an example of those, see the Lord of Blades from Eberron:RftLW), the Gynosphinx's LA allows it to cast any spell. A LA is not a regular Action, if it says it does something, then it does it, including casting any spell the Gynosphinx has access to, including Shield or Legend Lore. In effect, it changes the casting time from a spell from whatever it was to "1 Legendary Action".