GG97 is my name for the default closed-reference database used by QIIME. It is a subset of the Greengenes database (DeSantis et al. 2006) clustered at 97% identity. It is available from the QIIME github repository from the link below.
DeSantis, T.Z. et al., 2006. Greengenes, a chimera-checked 16S rRNA gene database and workbench compatible with ARB. AEM 72(7), pp.5069-72.