eckertt001: Getting the error from the installer, "Path contains illegal characters:."
The directory I am trying to install to contains a period. Other GOG installers do not exhibit this behavior, this is the only one I have found so far.
To the best of my knowledge, different GOG games use different versions of the installer - so some of them will accept paths that others won't. Others still will install, but will not like being placed in certain folders (Populous the Beginning, for instance, HATED being put in "Program Files (x86)" - probably since brackets are nonstandard characters for folders, and probably would have been denied in filenames back when it was made [but it's so long ago that I can't remember])
That said, TH works fine for me in C:\Program Files (x86)\GOG.com\Theme Hospital... so, I don't know. Probably because I used a much older version of the installer (I just downloaded one that is 2.0.0.5; the one I used to install was 1.0.0.4) - my guess is that the newer installers for DOSbox-enabled games is to shy away from nonstandard characters in the filepaths as these can cause some compatibility problems.
The obvious solution would be to just avoid using a filepath with a "." in it; back in the day the game would just be installed to C:\HOSPITAL and that was that.