The issue may be due to the spaces in the path to the GIMP executable not being properly sanitized. I just ran into another issue on github that I traced back to a sanitization problem on windows.
So, it looks like I鈥檓 going to have to revisit properly sanitizing strings on Windows once again 
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Progress can be tracked here, Problems sanitizing strings on Windows 路 Issue #492 路 darktable-org/lua-scripts 路 GitHub