The nonequilibrium microphase separation process taking place in polystyrene-block-polyisoprene (SI)/polystyrene (hS) blends during solvent evaporation was investigated by transmission electron microscopy to find a way to grow large gyroid single grains free of defects to be used as photonic crystals. Complex microdomain morphologies-gyroid, perforated layers (PL), and sponge-were found in a narrow region of the blend composition (complex phase window), and two different gyroid growth paths were identified for different blend compositions, both of them following a nucleation and growth process but resulting in different gyroid grain qualities. (1) 64/36 wt % (SI/hS): gyroid grains grow from sponge phase and reject part of the hS finally trapped as defects when small grains coagulate to form larger imperfect gyroid grains. (2) 67/33 wt % (SI/hS): gyroid and PL grains nucleate and grow consuming sponge, and then gyroid consumes faster growing PL. The lower nucleation density of gyroid grains results in defect-free large grains though surrounded by a small amount of accumulated hS domains. The second growth path, still requiring improvements, offers a promising method to grow large gyroid single crystals.