Leroux: For me it looks like this ("Keep Aspect Ratio" checked in the Advanced Settings of DOSBox Configurator, Window Resolution "normal", Scaling Engine "normal2x").
There you go. That's what it looks like with aspect correction on, and how it would have looked in a CRT monitor as the devs intended.
Resolution can be changed to anything you like if you are playing in a window and scaling is unnecessary if you set the resolution manually to the size you want. And it is absolutely useless if you play fullscreen, unless that you also want to apply a filter of some kind, which would mean you are a heretic that should not be allowed near any game made before the year 2000 :P
Garran: BaK ran at 320x200 natively, which is a 16:10 ratio.
320x200 has a 1.6 pixel aspect ratio. (...) However, when the resolution was used all displays were 4:3, and most users would stretch the 200 vertical lines to fill up the screen. Instead of perfectly square pixels, you would get pixels 1.2 times as vertical compared with the horizontal width on a monitor where the vertical width had been stretched to the edges of the monitor. Most graphic artists assumed this and adjusted their graphics accordingly, but not all did.
Source: http://nerdlypleasures.blogspot.com/2013/10/320x200-resolution-of-choice-for-ibm-pc.html BaK artists most definitely did.
And that is why "Keep aspect ratio" should be on by default in all GOG installs, unless they figure (or someone reports to them) that the game was not supposed to be stretched vertically.