So the patch is out and it promisses borderless windowed mode which I was hoping to see in game from the very start.
So how do I switch to it? Launcher in game folder just starts the game without any changes
and I couldn't find "Others - Configure Battletech" under "More" in GoG Galaxy Battletech settings.
Any tips?
Thanks in advance.

Check the Installation folder (usually %/GOG Games/Battletech). There should be a BattleTechLauncher.exe in there. Also make sure your game has been properly updated to 1.03, as there is neither the launcher nor the Galaxy entry with 1.02.

EDIT:
Scrap what I just said. The launcher is only available in the public beta channel, the regular 1.03 release removes it once selected.

The changelog is maybe a little messy and confusing in the sense not everything mentioned there applies to the "normal" patch but it specifies the launcher is available only in the beta branch, so I wouldn't say it's wrong.

Thanks for the explanation.
So that shortcut isn't launcher after all?
After a short investigation I found 3 options inside of it. One of which is borderless window.
But it leaves a top border of window visible.
So I hope when the actual launcher comes out it will be fixed.