| Origin | Code | Comment | Supplementary Taxo |
| Gallibacterium anatis | ISGan | | |
| Gallibacterium sp. | ISGasp | | |
| Gallionellaceae bacterium | ISGalb | | |
| Gamma proteobacterium | ISGpr | | |
| Gammaproteobacteria bacterium | ISGaba | | |
| Gardnerella vaginalis | ISGva | | |
| Gelidibacter algens | ISGal | | |
| Gemmata obscuriglobus | ISGob | | |
| Gemmatimonadetes bacterium | ISGba | | |
| Gemmatimonas aurantiaca | ISGau | | |
| Geoalkalibacter subterraneus | ISGes | | |
| Geobacillus kaustophilus | ISGka | | |
| Geobacillus sp. | ISGsp | | |
| Geobacillus stearothermophilus | ISGst | | |
| Geobacillus thermodenitrificans | ISGth | | |
| Geobacter bemidjiensis | ISGebe | | |
| Geobacter lovleyi | ISGlo | | |
| Geobacter metallireducens | ISGme | | |
| Geobacter sulfurreducens | ISGsu | | |
| Geobacter uraniumreducens | ISGur | | |
| Geodermatophilus obscurus | ISGeob | | |
| Gillisia limnaea | ISGli | | |
| Glaciecola agarilytica | ISGla | | |
| Glaciecola psychrophila | ISGps | | |
| Glaciibacter superstes | ISGlsu | | |
| Glaesserella parasuis | ISGpa | | |
| Globicatella sp. | ISGbsp | | |
| Gloeobacter violaceus | ISGvi | | |
| Gloeocapsa sp. | ISGlsp | | |
| Gluconacetobacter diazotrophicus | ISGdi | | |
| Gluconacetobacter xylinus | ISGxy | | |
| Gluconobacter suboxydans | ISGsub | | |
| Gordonia bronchialis | ISGbr | | |
| Gordonia polyisoprenivorans | ISGpo | | |
| Gordonia rhizosphera | ISGrh | | |
| Gordonia sihwensis | ISGsi | | |
| Gordonia sp. | ISGosp | | |
| Gordonia terrae | ISGte | | |
| Gordonia westfalica | ISGwe | | |
| Gottesmanbacteria bacterium | ISGoba | | Candidatus Gottesmanbacteria bacterium |
| Granulibacter bethesdensis | ISGbe | | |
| Grimontia hollisae | ISGho | | |
| Gulosibacter molinativorax | ISGmo | | |
| Gynuella sunshinyii | ISGys | | |