Hello,
Old Galaxy 1.x can work offline (if I remember correctly), but 2.0 can't. I cannot skip Login Form.
Why?

Sometimes Galaxy forgets my previous logging in, and then I'm not able to run client offline.

interesting. even Games Nebula (unofficial gog client for Linux) has online/offline switch and it never drops connection nor does ask for a login after such switch. That means if everything is working properly galaxy don't have to ask for login again.
I'd suggest to fill bug ticket if you want them to look into it (I'm not sure if gog tech staff reads forum).

i also would like offline mode i mean i understand if comunnity plugins or other games doesn't works but gog.com games should be working offline with time tracker

Offline mode is present, I have seen the choice sometimes at the start, but I'm not sure in what condition that windows is triggered...

I have noticed this issue whenever i restart the app but only sometimes or when having a slow internet connection. But overall I think it does have offline mode present, coz if I remember correctly the last time the launcher asked for my details was last week when I rebooted my laptop and was struggling with internet speed and since then it's working like a charm, I hope I didn't jinx myself right there haha