So If I'm a druid and I permanency a G.Magic Fang on my claws, does that spell go away when I revert to human form. I know the bonuses would not apply, but would the spell still be there, for the next time I was in an animal form?
Depends, but mostly yes.
Greater Magic Fang has two options. One is to hit a particular natural weapon, with a larger bonus. The other is to hit all natural attacks with a +1.
If option 2 was selected, then yes, you would keep it, because you are the creature, even when you are in human form, bird form, dino form, or bear form. And you would always get the +1 on all natural attacks, no matter what form you were in.
If option 1 was selected (and claw was you choice), then the spell would basically suspend until you had a claw attack again, because the target is not the attack itself but you. At least until someone dispels your awesome glowing death paw!
BTW: If you did select the first option for Greater Magic Fang, like Magic Fang, it only affects ONE (singular) natural attack, not attack type. So for your claws (plural), you would have to have it cast two times.