Head northeast from Edea’s Orphanage to the Centra Ruins (where you fought Odin). Take Squall and Quistis along, and have one or both at critical status in order to use limit breaks. Head for the 3rd screen (the one with the Tonberry statue in the center of a circular platform).
Once there, start taking out as many Tonberries as you can. You’ll gain 0 EXP (and Chef’s Knives) for your efforts, but after the 20th Tonberry you kill, there’s a chance of meeting the Tonberry King. This guy seems to appear randomly, so you might have to rack up quite a few dead Tonberries before actually meeting him. You don’t really have to take down 20 of them in one shot; you can always head outside to save before continuing.
GF TONBERRY KING
Draw: Death, Curaga, Full-life
Draw a few Full-life spells from him then cast Meltdown and unleash Squall’s Limit Break. He should be dead by then. Just watch out for his magic spells and heal/revive as necessary.
Once defeated, you’ll get GF Tonberry.