The Garuda Girl was only mentioned in Part 2 and has made no actual appeance yet.
After having dealt with the attacks on the human capitals, Luka's companion realizes that they will need the Garude Girl to reach Hellgondo. Actually getting the conditions ready to summon her makes for the remainder of Part 2's main questline, as the party first must get the Six Orbs needed to summon her (including one in another world because this version had been destroyed by Astaroth).
As the party attempts the ritual, the summoning fails, and they're informed by Nero that it is related to the Snow Continent that disturbs the flow of the mana in the world. Once the core of the issue is dealt with, the party stands finally ready to summon her.