A centrosome-localized multiprotein complex composed of gamma-tubulin and other non-tubulin proteins assembled into a ring structure that is thought to be the unit of nucleation at the minus end of a microtubule. Gamma-tubulin small complexes are thought to be the core repeating units of the ring. [ http://www.ncbi.nlm.nih.gov/pubmed/12134075 http://www.ncbi.nlm.nih.gov/pubmed/17021256 ]