I installed G'MIC 18.104.22.168 on my Windows XP today and when I invoke GIMP I get the message:
"The procedure entry point gzdirect could not be located in the dll zlib1.dll"
This message appears once, then I briefly see a DOS-style window appear, then this message appears again. G'MIC does not appear in the Filters list.
I installed G'MIC in GIMP 2/lib/gimp/2.0/plug-ins.
This worked with previous versions.