What's the benefit of logging in to XBox Live, PSN and other kind of platforms through GOG Galaxy 2.0 on PC? It only shows me trophies. Also it appears that i have Kingdom Hearts III and Overcooked (and i'm pretty sure that i don't own 'em).

Sorry for my ignorance. Please enlighten me about this feature.

Try to think of GOG Galaxy 2.0 as a library, but also as a community and social space. They are there to allow you to see what your friends are playing and connect that way.

I thought logging into your MS account means you can play the games you own on the PC, like Gears 5, and Sea of Thieves. You can't play those titles?

Currently only what you mentioned is possibe but further down the road gog plans to get the community features up and running. And in case of XBOx Live, you will also be able to install and play games that are in the "play anywhere" program or are PC only. And my guess is that rather sooner than later people will be able to play Playstation titles on their PC via Sonys streaming service.
The idea behind all that is, that regardles of the plattform (and this does not mean Hardware anymore) you bought your games on and/or keep your gaming contacts, Galaxy is your tool of choice for your "gaming life". With gogs store always beeing present.