Is there a way to get this to run in windowed mode or in a 4:3 aspect ratio? I cannot get it to run other than in a 16:9 ratio and everything looks stretched on my monitor regardless of what I set the graphics resolution to. I've tried ALT-ENTER and also adding -window to the command line but it always starts in full screen mode.

You need to download Call of Cthulhu Widescreen Fix (not sure if it works on GOG version though)
https://pcgamingwiki.com/wiki/Call_of_Cthulhu:_Dark_Corners_of_the_Earth#Windowed

Thanks. Doesn't seem to help though.

If you just want to run the game windowed, you could use DXWND:
https://sourceforge.net/projects/dxwnd/

It's not the most intuitive program to use, so if you are having difficulty making it work I can try to write up a tutorial for you.