I think that in the early days before Sony had many lenses of their own yet they wanted to have something to offer so partnered with Zeiss until they could develop a fair number of their own lenses..... Just a guess, anyway.
Sony's GM lenses are their top-of-the-line (and most expensive) lenses, and then comes their G series, then after that lenses without any particular designation. My 135mm f/1.8 GM is indeed outstanding, a really gorgeous lens, and my G lenses (90mm f/2.8 macro and 200-600mm) are really excellent, too. I also am quite happy with the non-G lenses as well, and although they are less expensive than the others, they certainly perform well and do what I need.
I have Sony's RX10 IV, which is a "bridge" camera which has a zoom that ranges from 24mm-600mm on a one-inch sensor, and that lens is a Zeiss. Also the RX100 VII, a compact camera with a shorter range up to 200mm also on a one-inch sensor has a Zeiss lens. Great quality, excellent sharpness on both cameras!