Ever since i upgraded my Windows OS, i been getting black borders on each side for old games, i try to configure them in GOG's Screen settings, but it doesn't seem to work. I also configured with my graphic card settings and that doesn't help either! I think its Windows 10 being the current problem right! I hope there will be a fix for that issue soon!

What video output are you using in Dosbox?

Opengl

What you can do is open the game's dosbox%.conf with notepad and set fullresolution=desktop.
You can't use gog's configurationtool for this as the resolution you put in there only applies for the windowed resolution for some reason.