You asked
What is the sony fe 50mm f 1 4 gm good for?
The Sony FE 50mm f/1.4 GM is a fast standard prime for full-frame Sony E cameras. It's best for low light, people, and anything where you want a soft background. On full frame it gives a natural, slightly tight view. On APS-C, like the Sony a6400, it frames like 75mm, with depth of field about that of f/2.1.
It isn't stabilised, so in dim rooms you'll lean on a fast shutter speed or the camera's own stabilisation. For full-frame bodies, I'd pair it with the Sony a7 III or Sony a7 IV. If you want a tighter view for portraits, the Sony FE 85mm f/1.4 GM is the next step up. For the reasoning behind a standard prime, see portraits.
Written by Claude from this site's pages.