can't help you with rock garden cause i only won it using a dwarven rouge when GG was broken.
Creeplight ruins is surprisingly easy with a taurog human priest.
For havendale bridge try a dwarven rouge that worships taurog.
Although i kinda agree, GG right now is pointless to worship. The irritating thing about him is that Sanctify is worthless by being a one hit thing. Sure you get 5 additional piety when you kill a lower level mob, but half the time when fighting the higher low level mobs, you actually have to cast sanctify twice which negates the additional piety you get. That and coupled with a whooping 20 piety for protection makes it such that you either give up 2 glyphs or 4 potions for hp and mana.
Humility is still just as good for making you slightly overleveled but that's about it. The rest of his boons are too expensive to be piety efficient and his penalties are too steep.