From my memory, I'd say that your GI solution has too few photons + too low accuracy + and too low a exponent value.

A way to get a visual clue, in regard to photon count, is to set the exponent downt to "1" (in order to see the photons more clearly). And reduce the GI radius if needed. The point is to see the small "spots" that comes of this. Looking at the small spots you get an idea as to where the GI photons hit.

Not sure what an optimal solution would look like with GI, but go for a solution that has a decent distribution of photons onto your model.

Secondly; Fiddle with the radius setting. Not sure what is optimal, but I'd say, go for a radius settings that makes the photons overlap each other. If the radius is set too high, I guess the light will turn up wierd, but then again I'm not an expert.

With photon count and radius set. Increase the accuracy to get a smoother rendering.

Last (I guess), turn the exponent up from "1" to "2" or around that. I think the "normal" light exponent setting is "2" for natural sunlight.