If it's gg_13_5 release, they provide a map to GenBank accessions on their site (gg_13_5_accessions.txt.gz). You can further map those to NCBI taxon IDs. Be aware that a large fraction of these get "bacteria: environmental sequences" etc. useless taxonomies. There are some maps on the NCBI ftp too, although I can't remember where exactly and what kind..