go-task pkgconfig -- gtk+-3.0
task: [pkgconfig] /home/thomas/Projects/SolusPackaging/common/Scripts/epcsearch.py gtk+-3.0
pkgconfig(gtk+-3.0) found in: libgtk-3-devel
So that dependency is the libgtk-3-devel package
Easy to find with the go-task commands and/or epcsearch.py script from the packages repository as used in the above example. Unfortunately a bit overkill to install them if you're not packaging for Solus.