Hmm, thought of this last night, maybe agnostic could get a +%GP birthrate, based on pop. Since they will never be able to benefit from pacifism, the most awesome civic a phil/ind leader could get, and, until you get free religion and or monarchy it will be pretty hard to get your population up.
a +5% bonus in each city for each pop would be cool. Early game this would be like 25%, pretty modest, but when you get to late game it would be huge +100% at pop 20. That may be too high, but considering how many GP the grigori would normally produce in a game it may not be out of line. Maybe you could cap it at 50 or 100%.
Or, you could just give them a flat +GP% at the outset.
Also, once you introduce the crusades, the grigori should have some sort of weird diplomatic reaction to the crusading civ.