Defeat a nearby Togre (ネクロバルサ) to get the item Evil Horn (邪鬼のツノ). This is tough to do at level 50, so it's best to leave this for when you're higher level or get higher level allies to help you defeat it. They can be found around E-5 in Becon Valley.
Talk to Gragado again and then return to Glen Castle. Talk to Ranbaba to clear the quest.
This unlocks Special Training, which can be switched to by selecting the option from the Dharma Priests. Instead of Experience points, you will be earning Special Training Stamps which, after enough of, you'll gain a skill point. You do not earn any EXP while in this mode, so it's best to switch to it once you hit level cap.