I've copied the gswin32c.exe to the ..\Portable GIMP\App\gimp\lib\gimp\2.0\plug-ins folder, but I still can't open up PDF's with GIMP.
I get a trio of error messages:
PDF document Message
Error starting ghostscript: Failed to execute child
PDF document Message
Could not interpret ...
GIMP Message
Opening ... failed:
Plug-In could not open image
I found the solution: http://understandinglimited.com/?p=7