Is it possible? I don't like those two black squares on the screen :( maybe the image on screen could be smeared

Not unless someone wants to write a mod for it. There's currently nothing out that can do that, and I doubt it will ever be made.

Sure there is.

Hit win+r to open the run dialog, type %APPDATA% and hit return. Look for a folder called GOG, or create one if there isn't one already. Inside that create a folder called Silver, and inside that create a file called config.ini, and type into it

[Video]
ScaleMode=0

That'll give you fullscreen, but it will be rather horribly stretched on a widescreen display...

Well yes, you can stretch nearly anything, but for a true resolution increase, I think it's a pipe dream.

Yes, that's true. I read the question as just wanting to get rid of the bars even if that meant stretching the screen, which is possible, but if the question was whether you could increase the real resolution, it's not.