Heyho.
I can't fully use the client because it doesnt support my screen resolution.
A workaround could be to run it in a windowed mode (I find this fullscreen stuff annoying anyway), but I dont find a configuration tool, or at least a .ini data.
Please help me.

Edit: my resolution is 1280x720 And Scaling is set to 150%

That's odd, the client is windowed. I'm not even aware of it having a full screen option.

By the way, are you sure your resolution is 1080x720? That's a really odd combination, it's much more likely to be 1920x1080 or 1280x720.

EDIT: Your screenshot is 1280x720 but I have no idea why the client is appearing so huge on the screen. Do you have windows set to scale things up for small screens perhaps? Try right clicking on the client shortcut, going to compatibility and choosing disable DPI settings.

If your screen really is 1080x720 (And I'm also thinking you may be looking at 1280x720) you may want to bring this up in the Development sticky on GoG Galaxy.

https://www.gog.com/forum/general/gog_galaxy_official_discussion_thread

That thread is monitored and it may be an issue that needs to be resolved.

You guessed Everything right, but the solutiion didnt work

Solution:
-reset scaling to 100%
-maximize window
-set scaling back to original value

Thank you for your suggestions :)

It's probably because of your scaling - why do you need 150% on such small resolution monitor?