Doesn't look like a good candidate for GR. You should save GR for flat, diffuse, nearby light colored objects passing indirect light to each other (eg rooms with white walls and such).

GR has to do a huge amount of extra work, but won't look any different than much faster rendering lighting types, if used in inappropriate scenes.

Have you tried Ambient Occlusion?

Vincebagna is right  on with careful use of custom "User" render settings to minimize render time.

Excellent modeling!