Hello ViviQuen.
You are indeed supposed to get it from Gilgamesh. If you got the S-rank in disc 3, that means you didn't have the mod properly installed. Most likely, you are playing the PC version of the mod and have the Moguri mod installed and didn't let AlternateFantasy have priority over MoguriFiles (and didn't notice the mod was malfunctionning until now).
In Memoria.ini, you need to have "AlternateFantasy" written BEFORE "MoguriFiles" in the list of mod folders, to ensure compatibility.
Sorry for the delayed answer. Since you were new on the forum I didn't get notified your message.