When rendering, it's a good idea to have Detail Blending at 1, though it is not for GI specifically but for other details of the scene. For GI issues like this one, as stated, a higher GI Blur Radius is a good idea. 300-500, even 800 (though I don't now how much different the effect is at those higher settings, and there may be detail loss in the lighting at that point). Another setting you can fiddle with is the Sample Quality. If you increase that, separate from GI Relative Detail, you can often improve things. For example a setting of GI Relative Detail 1, Sample Quality 3 or 4, and GI Blur Radius 500 might give good results.
- Oshyan