Did you try the gmic_gimp from the 'buntu PPA
https://launchpad.net/~otto-kesselgulasch/+archive/ubuntu/gimp It is old-ish but might suffice.
The problem with 'buntu 18.04 is the QT version. The versions on the gmic site use a more recent version of QT than is available in 18.04. The solution, compile your own.
The other solution is use an appimage, which for various reasons, not for me.
This is gmic_gimp_qt 2.5.7 from about a week ago
This does work, unzip put in ~/.config/GIMP/2.10/plug-ins (permissions should be set but check anyway)
Looks like this https://i.imgur.com/dS4nFgg.jpg
There are other reasons why gmic_gimp might not work, usually missing one of the QT packages, often libqt5widgets…