It seems the dosbox emulator doesn't care if i check the "Keep aspect ratio" box it runs the game in fullscreen anyway. Is there a way to run the game in 4:3 letterbox in fullscreen?

For most GOG games to preserve the aspect ratio, I ususally have to open the dosbox .conf file in the game's directory, find the following values, and change them to the ones shown here. Feel free to experiment, though.
fullresolution=desktop
output=overlay

aspect=true

Thanks for the help but it seems it doesn't work because i still don't have the black bars on the sides. I don't know if the graphic setup utility that comes with Galaxy messes things up and overrides the settings in the dosbox config file.

Klappis, what resolution are you playing in?
Did you try picking a 4:3 resolution?

Look in your GPU drivers for scaling with aspect ratio.

Tried to pick 1600x1200 if that's possible?

OK, that should give you a proper 4:3 image. At least I got a proper, undistorted image when I picked a 4:3 res.
The next things to check should be:
1. scaling mode on your GPU drivers (as Gydion suggested)
2. scaling mode on your monitor (some monitors have different settings for scaling, which can screw up a perfectly correct signal coming from your GPU).

Play aroud with different combinations of those 2. You might also try setting one or both of them to 'no scaling', 'original size' or something like that.

Thanks a lot!