I got G'MIC working! Thanks for all the help, everyone!

Because I had messed with several things, I uninstalled G'MIC and GIMP then did a clean install of GIMP as before, in the Program Files directory. Using the automatic G'MIC installer, I put the G'MIC files in C:\Program Files\Gimp 2\lib\gimp\2.0\plug-ins.

Edit: The installer did not complain about duplicate files.

Previously, I had made a Plug-in's directory in the the main GIMP directory. That was probably the problem.

It probably would have also worked fine if I had let the installer put the files where it wanted, in my Windows 7 account tree.

I played with G'MIC a bit to see if it was working. It looks like all I hoped for.

Thanks again for helping me.