1. How do you know if a lens is a G Master lens?
2. Are the G Master Lenses the ones with the widest apertures? Is there much quality difference between G Master lenses and non-G Master lenses?
3. Is there anything to look for in a lens to take advantage of the eye focussing features in the A7R3?
4. Are there 3rd party lenses that meet the same quality as the G Master lenses?
1) if the part number starts with “FE” and ends with “GM” it is a g master lens. Don’t be too concerned with what it’s called, though. Price and capability matters.
2) generally speaking yes, for a given focal length, Sony’s GM lenses have the widest apertures within their lineup. Regarding quality... probably, but that’s a tough question and depends much on what you mean and what your priorities are. Does quality mean build? Does it mean sharpness and distortion?
3) Not really, but Sony lenses seem to work better than non-Sony lenses in this regard (I have the Zeiss batis 1.8/85, and the Sony FE 85mm f/1.8 (which I borrowed to try) focuses faster in low light in my experience on both rII and rIII.
4) Almost certainly. For example the cosina manufactured Zeiss 1.4/85 ‘otus’ is arguably higher quality than the Sony 85mm f/1.4 GM, but it doesn’t come in a native mount nor does it have autofocus, and it costs 1.8 times more. Some of sigma’s offerings may also play, with similar caveats. If you limit the scale to native mount lenses available today, GM is probably the best quality.