I tried setting the resolution to 640x480, and use "original aspect" through the gog.com Galaxy app, but nothing seems to work.
The game is stretched to the 16:10 of my modern-day monitor :(
If your graphic card doesn't know to fill 640x480 with additional information to the left and right for it to look correct on a 16:9, 16:10 or 5:4 display it will look stretched, no matter the "aspect=true" setting. With both Nvidia and AMD graphics cards there's a GPU scaling setting, that you can set up so the GPU scales the incoming resolution to the native resolution of your display using the correct aspect ratio.
line Tomy mentioned together with the
line, makes Dosbox do this by itself. The 0x0 tells Dosbox to output fullscreen to the resolution of your desktop while the aspect=true makes dosbox add the correct black borders. Personally, I prefer the correct resolution of my monitors (using the 0x0 line in the .conf file) to the rather blurred results you get by using GPU scaling but you can't argue about taste. Try what you like most. Additionally, I like using a CRT-shader that comes with a Dosbox SVN build that makes the output look close to the real thing.