CONTAINS SPOILERS

To obtain the glintstone key required to open the South Raya Lucaria Gate, you must first travel to the west of the gate (view image for location on the map). There is a Glintstone Dragon Smarag sleeping next to a rock formation. If you choose to skip this battle, you can simply ride horseback and gallop behind and to the right of the dragon. Pick up the key and sprint away fast.