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.

Exactly.
There is a "Launch BATTLETECH" shortcut inside the game folder but it just starts the game normally.
So the changelog for this patch (I don't use beta channel) is wrong.

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.