Look, you can't rely on gfs all of the time. I don't use them much (except for supportive ones) in battle. I just junction them so i can junction magic, refine magic, and use other abilities. Here's a couple of examples as to where you will need more than GFs:

Ultimecia will place seals in her castle so all you can do is attack. (You can unlock them, but it'll be hard seeing as you only used GFs)

And a possible spoiler: (SPOILER)Adel will try to absorb Rinoa's powers and she'll be inside of her, if you use gfs they will hurt rinoa too. If Rinoa dies, it's game over.

Junction everything right to max out their stats and it'll be awesome.
My characters have ultima junctioned to their elemental defense so they recieve no elemental damage. Their stregnth is maxed out to 255 so they can do up to 9999 damage in a regular attack. Junction Aura do stregnth, draw at the island closest to Heaven/Hell untill you have lots of magic. Use squall, rinoa, and zells limit break. Get all of the GFs and strongest weapons.