The csd drawn apps, like Gedit you posted a screenshot of, are comprised of more than just a Window size. The containers within for boxes, controls, toggle buttons... Those all have a defined width.
If using Zorin OS LITE (15), you can use Mousepad, Pluma or Xed, all of which can get pretty narrow on my system, no matter which theme i use.
I personally avoid Gedit, as I find it clunky and the forks Xed and Pluma are much more user friendly.
If you like, I can post a css snippet that you can paste into ~.config/gtk-3.0/gtk.css that MAY help you narrow down gedit a bit.