Don't forget that there are still other members of God Hand. Griffith may want to be the ruler of his kingdom, but the purpose of God Hands is to merge the astral world and the living world. Expecting some fighting within God Hands if indeed Griffith is a good guy.
I also find it iffy that the monsters appear to chase people away to Falconia, This massive gathering of souls in one place may have been a plot from the start to stage a big eclipse for all the God Hands to be born in the living world.
In this current stage, Guts is no match for God Hands so the answer to whome he will get his power from will determine the story moving forward:
1) Power from the Elf King: Guts & Co. destroy all the Gods Hands, including Griffith by themselves --> Shounen standard plot
2) Power from Griffith: Griffith turns good, and Guts tragically has to give up his revenge and pride to fight along side Femto against Void's army --> very interesting plot twist, Guts is a tragic hero
3) Power from Void: Griffith wants to take control over God Hands by sacrificing all those people. Guts has to ally with with Void, become the devil himself to stop it --> most tragic situation out of the three. But it seems too out of Guts' character to do that.