It is a QT issue. Both zips from www.gmic.eu (Debian sid / 'buntu-cosmic) require a more recent QT than is available.
gmic_gimp_qt needs to be compiled against an older QT5.
This works in a Mint 19 VM Gimp 2.10.8 from the otto-kesselgulasch/gimp PPA.
However plenty compiler warnings about cimgz expansion and the file size is suspiciously small at 7.7MB
The usual: uninstall the old gmic_gimp, unpack the zip, put gmic_gimp_qt-mint19 in ~/.config/GIMP/2.10/plug-ins, check the permissions.
In addition, I would be inclined to rename the old gmic profile ~/.config/gmic as a back-up and start afresh.