Windows bar? This just makes me feel like I'm on a Internet page with something else than Chrome.
Please look at Stream, Origin, Uplay and Rockstar Games clients for a complete look.

Awful choice of font with caps.
Some have different letters for their name like BenAna, but instead it shows BENANA.

When I exit my GOG Galaxy client, I want to exit it.
Instead it falls into the right corner with the many other small icons that are hidden.

Achievements full of spoilers!
You should have those achievement with character names and such.. just say "Secret" as name or\and description!

They only minimize. To come to their rescue, they are not alone in doing this. However a checkbox in the preferences about the preferred behavior of the close button would by far be the best.

Menu bar?
Is this really necessary? After all there are only two big things: Library and Account. Please make a proposal what entries there should be in a menu. (I'm also running my browser without an explicit menu bar.)

I feel that the Windows bar is unnecessary.
Best solution is to have "exit" and "minimize" in the design of client software.
Check the image below
There is enough room on the right after the username to have "exit", "minimize" and etc.