Hello Patrick,
The installer provided on the G’MIC website is known to be not working with current development version of GIMP 2.9.8+. It used to work before, but it’s not the case anymore. I suspect the GIMP team has made some changes in the plug-in API or whatever that breaks the compatibility of the plug-ins between GIMP versions 2.8.x and 2.9.x.
On the G’MIC website, we currently only provide plug-in built for the stable version of GIMP (so 2.8.x).
I know some good folks are providing binaries of the G’MIC plug-in that is working with GIMP 2.9.8, so you should check what they propose on their website.
I’m aware of two sources to get a working G’MIC plug-in for GIMP 2.9 :
- Samj’s website : http://samjcreations.blogspot.fr/2018/01/version-de-developpement-gimp-299_11.html
- Partha’s website : https://www.partha.com/
I cannot ensure it will work as expected, I don’t have GIMP 2.9.8 installed on a Windows machine, I’m sorry about that. But for sure, they probably will be glad to help if you get troubles with what they propose.