It would appear that the latest versions of Krita since 5.0.0 changed the way the g’mic plugin is handled.
Previously there was a package g’mic-krita in the repos.
According to the Krita website it now requires g’mic-qt which is not in the repos and when trying to launch g’mic inside Krita it gives the error “The GMic plugin is not installed or could not be loaded.”
This happens even is g’mic is present on the system.
The Krita website offers flatpak versions, but I would prefer to get everything working via pamac.
Could g’mic-qt be added to the repos in a fashion that it would make Krita work as before? I am not sure how this should be approached correctly.
For a while, use the AppImage version from the Krita website, put it in your home directory, chmod u+x AppImage, run it.
And you can switch to pamac version when the Arch official upstream has the Krita 5 packaging fixed.
I expect this can take a while, since Manjaro official repository takes from Arch official repository and there’s several testing (unstable and testing branch) before making packages available (stable branch).
I don’t recommend building G’MIC-Qt on your own. For cmake build system, building and installing may be fine, but it will be problematic when you try to uninstall it.