This is about the gmic_gimp_qt plugin for Ubuntu 18.04 (also Mint 19) which is a LTS favoured by many who do not want to re-install their OS every six months.
As of today 14 April 2019
I can compile in my regular 'buntu 16.04 using the last of the PPA pre-Gimp 2.10 (2.9.9) - no issues.
Same with 'buntu 18.10 and the default Gimp 2.10.6 packages
However, failure, this with 'buntu 18.04 and the PPA gimp 2.10.8 packages.
Believe me, libgimp2.0-dev package is installed. Been like that since the last gmic_gimp_qt version 2.5.5 compiled 23 March.
Tried all I know without effect. Your down-loadable packages do not work here with PPA for 'buntu 18.04. Both sid and cosmic packages fail - wrong gimp version - wrong QT version…etc.
Bound to be something simple, or even better can you provide 'buntu 18.04 (bionic) packages.