The plug-in binary provided on the website is not compatible with GIMP 2.6.
The plug-in API has been changed between GIMP 2.6 and 2.8. If you need the G'MIC plug-in for GIMP 2.6, I'm afraid you will have to compile it by yourself.
It should compile and run without errors for 2.6 by the way, it is just that the plug-in binaries are not compatible.
@sallyanne you have another issue, I think you have tried to install the cli interface of G'MIC instead of the plug-in.
If you are interested only by the plug-in, you should use the installer available here :
http://gmic.eu/gimp.shtml