The specification for sgis_gen_mmaps explicitly states that it only works for n-dimensional textures. I checked the cube-map spec, but it doesn’t really even mention sgis-g-m.
It would be nice to enable sgis_gen_mmap on a cube-map used as a procedurally-generated environment map. Maybe it could be enabled per-face or something, and each face gets automatic mipmap generation independently.
Also, is sgis_gen_mmaps faster than gluBuildMipmaps? If the texture format/size is already set and allocated, then I would imagine so. Is it fast enough to use per-frame?