Gabby is easy if you do it right.
Give edge the best defense you can, level up No Guard to max, and equip an anti paralysis amulet.
Give Riemi the best attack gear available, and give her crimson squall and/or savage sparrows with critical hit on support if you have gotten it.
Make sure you use Sarah as your healer, turn off all of her spells but her healing and resurrection spells, and remove all of her chain attacks.
You can use whoever you want for the other party member, I prefer Meracle for damage, but that's up to you.
Taunt Gabby with edge, and run lap around the outside of the battlefield with Gabby chasing you, and let your AI party members attack him.
Pretty much you should be able to kite him like this until he dies.
If that doesn't work, go level more. On level 6 of the ruins you will find crabs and metal scumbags who drop a hell of alot of exp.
I beat him around level 120, but if you need to get higher there is no shame in that.
Hope this helps, and good luck. :)
We'll miss you George.
PSN:Puzzleface
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My friends call me Hadoken because I'm down-right fierce
