The G subscript indicates loudness computed from critical-band levels

Evidence

The index G at the unit “sone” is added as a hint for the user that the loudness given in this value is produced using the critical-band levels. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1271

Categorization

• DEFINE: Explains the meaning of the subscript G in soneG and distinguishes critical-band-level loudness from other measures.