Gippal can be found in a variety of places throughout the game. The first time you encounter the young Al Bhed is Djose Temple in Chapter One. He will interview you for a position in the Machine Faction and, should you accept, will bestow a letter to Nhadala, head of the Bikanel Digs. This is where he reunites with both Rikku and Paine, albeit in different ways; Rikku he greets with a jovial, "Well if it isn't Cid's Kid", while Paine receives a shocked "--YOU!!". Paine urges him with her eyes and responses to pretend they don't know each other. He quickly catches on, though seems less than pleased about it. Oddly enough, Rikku and Yuna don't think to ask either of them about this meeting. For the remainder of the chapter, you can find Gippal in the left bedroom of the Temple. He isn't very chatty, though he will ask if you've fulfilled all your wildest dreams through digging.
The second time you can see Gippal is in Chapter Three. Fiends are rampant throughout the temple, and the Gullwings offer to help out (for a small fee, of course). Gippal shoots down their offer, and declares that not only can he watch his own back, but Spira can't beg Yuna for help every time there's a little scrape. He teases Rikku relentlessly here, calling her "Little Girl" and joking that they once made quite the couple. It appears to only be in good humor, however.
By the end of Chapter Three, things aren't quite so smooth for the Gipster. You'll find him lurking in Bevelle, and if you follow him he will unwittingly lead you to the inner sanctum where Nooj and Baralai are waiting. There is an extended scene in which an argument breaks out, and all three pull their guns. It is a shadow of the Den of Woe scene from their history, and just as happened then, Paine screams for them to stop. It is then revealed that Shuyin has been inhabiting Nooj's body the entire time, and now has decided to move into Baralai's. Gippal and Nooj take off after him, while the girls fight a rather fierce Malboro.
Shortly after Yuna finds herself mysteriously on the Farplane, she encounters Gippal again, this time with Nooj. They're in pursuit of Baralai/Shuyin, and they hand her two spheres to return to Paine. Nooj comments that they are their wills; Gippal quickly reassures (in an amusing, almost peeved tone) that they are not, and that he and Nooj have the situation under control.
Later, in Chapter Four, Shinra's broken commsphere will pick up a transmission from Gippal and Nooj on the Farplane. Gippal evidently fixed it, at least for one way transmission, and the Gullwings overhear a short blurb in which Gippal scolds Nooj for being so ready to die, and then asks about LeBlanc. Much to LeBlanc's disappointment, the transmission cuts off.
When you finally arrive at the Heart of the Farplane in Chapter Five, Gippal will greet you shortly in. He has been injured, and Nooj has gone on ahead with him. LeBlanc and her goons are assigned to watch over him and the Gullwings race to find Shuyin and Nooj.
When the big battle comes about, Gippal and LeBlanc reappear, ready to get back into action. Gippal confidently tells the group that Vegnagun can be defeated because it is little more than a machina, and anything created by people can be destroyed. Rallying on, the groups split up to take on the massive behemoth. Eventually it is up to the Gullwings to save the day, and so they do.
Finally, there is an extended FMV ("Normal Ending") that shows Gippal, Baralai, and Nooj giving a speech to Luca Stadium. They regret their actions of division, and promise to make Spira a brighter tomorrow. Gippal has the closing words; he announces that they must truly thank one special lady, Yuna, and he joyously tells the Spirans that she said she was "going home".
You can also find Gippal recorded in various spheres. There are eight Crimson Spheres and a special Gippal's Sphere. For more information on obtaining this secret sphere, click here.