G Master lenses weren’t around when E-mount started because Sony focused on making compact/lightweight lenses for mirrorless since A-mount was pretty much complete.
The focus on compact/lightweight glass is why Sony released f/4 zooms, FE 55mm f/1.8 ZA and FE 35mm f/2.8
The release of the a7RII necessitated the development of G Master glass to satisfy professionals.
Similarly, the release of the Sony a9 required lenses like the FE 100-400mm f/4.5-56 GM OSS be developed for sports and press.
Sony is trying to make their E-mount system complete thanks to the a9, which is why the development of the FE 400mm f/2.8 is actually ahead of schedule.
G Master glass is 100% developed by Sony with no compromises to get the best possible performance and bokeh.
G Master lenses are also designed for video functionality and the lenses coming were designed with the a7RIII and a9 in mind.
G Master lenses are Sony’s most durable and robust lenses.
Operational feeling, sound, image shake and focus movement are all taken into consideration for movies with G Master lenses.
Sony claims their G Master designs expand on what Zeiss has done for Sony in the past with lenses like the A-Mount 85mm f/1.4 from Zeiss.
Sony won’t flat out say their G Master designs are better than Zeiss and leaves it up to consumers to decide.
The only clarification given about the difference between Zeiss and G Master designs is that Zeiss designs focus on resolution and contrast, while G Master designs focus on resolution and bokeh.
G Master lenses are designed to a higher standard than G lenses like the excellent FE 12-24mm f/4 G and FE 90 f/2.8 Macro G OSS
It’s unlikely we will see G Master 35mm or 50mm f/1.4 lenses for at least 2 years since Zeiss versions of these lenses were released fairly recently and they are excellent.
The FE 100-400mm G Master is the only G Master lens currently made in Japan. The Others are made in Thailand at the moment.
Sony somewhat acknowledges that the very popular FE 85mm f/1.8 can rival the FE 85mm f/1.4 GM, but says the feal of the lenses also plays a part.
Sony thinks that the use of the 1.4x and 2x teleconverters will help fill gaps when the FE 400mm f/2.8 is released because the teleconverters don’t slow down the lenses ability to AF at all and you simply get more depth of field.
Lens development should outpace camera development in the coming years and lenses will become outdated after 10 years.