If you missed any GF on your way through the game, you can get them during Disc 4 at Ultimecia's Castle. After defeating Sphinxaur, choose to unlock the Draw ability. You'll then be able to draw the following GFs from the corresponding Bosses.

Unlockable	How to unlock
Alexander	Catoblepas
Carbuncle	Krysta
Cerberus	Gargantua
Eden	Tiamat
Leviathan	Trauma
Pandemona	Red Giant
Siren	Tri-Point