If GOG's version is working in DOSBox you can tweak the .conf file to force the game to go widescreen without bars. Sorry, but you'll have to do some legwork:
1. Locate the .conf file that the DOSBox install specific to your game is using. For GOG games, usually it's in the game install directory, renamed to something like dosbox_simcity.conf.
2. Back up the file.
3. Examine the file, it's fairly well documented by the DOSBox devs. Puzzle it out on your own if you can, this is essential knowledge for playing old games anyway. It'll put hair on your chest.
4. The settings you're most interested in are probably fullscreen, fullresolution, windowresolution, output, aspect, and scaler.
5. An example of what you might want to change these to:
fullscreen=true
fullresolution=(your custom resolution)
output=(experiment with this for best performance. a cheap netbook will probably do best with surface, but this limits scaling.)
aspect=true (or false, whatever works in combination with your resolution)
scaler=(experiment with this on higher resolutions. "none" to start with. IMO, on a larger monitor, you're better off using a scaler and playing the game in a window, it looks far better.)
You will probably have to do some trial and error to get rid of those bars, play the game in a window, or get over the bar-hatred. That's life.
That promo screenshot that GOG provides is, IMO, doctored for general appeal. I guess you could fire up SCURK and make something similar to it, though.
-- I'm not responsible for your computer exploding, if and when this happens.