I guess by around late 43/ealrly 44, but there were a few leftovers of the G1 through G4 series (all rather similiar w/o the 13mm bumps, different radioset etc). I did check once the ratio, in mid-43 it was ca 50-50 between G-6s and earlier Gustavs, G-6`s amount increasing steadily. Keep in mind the G-6 was only in production from February `43 onwards, and by the automn, lot of improvments were made to it compared to the spring - higher power, better Erla canopy, MK 108 begun to be fitted etc...
The G-1 and G-3 were fairly few, they were special high altitude fighters. G-2 and G-4 being more numerous, ca1500-1500 being built of both, they differed only in radio set and in having non-pressurized canopies.