When I started, Qt required a preprocessor (moc) for the build process. Gtkmm did not do, and use libsigc++. This seemed like a clear sign to me that it was technically superior.
22+ years later I would say that both Qt and Gtkmm have their own pros and cons. They have both run into pretty deep issues with integrating their own drawing model into the windows, macOS and X11 models. They both provide things that the other does not.
At this point, I don't see one as better than the other, and for a long established application like mine, the idea of switching is essentially unthinkable.
I did the same as PaulDavisThe1st; gtkmm is an excellent C++ interface for Gtk, such that I wonder why they even bothered to create Vala except for NIH syndrome. Qt had similar issues in the past: it wasn't pure C++, it was C++ "with extensions"; you had to use a custom compiler (moc), and you had to use Q-versions of everything the STL provides (QString, QVector, and so on).
Of course things were different more than a decade ago: C++11 did not exist yet or compiler support was lacking, and Gtk and Qt couldn't wait for it to catch up.
Yes you had to use qmake. 99% of the developers never even touched moc, it was baked into the build system and gave you things like signals and slots.
I remember doing this https://github.com/nurettin/trendline_analysis_qt/blob/maste... (Qt gave me graphicspathitems that I could do rubberband select on an accelerated qgraphicsview more than 15 years ago. I loved gtkmm and standard c++, but Qt was simply amazing. And nobody forced you to use Q*containers. I always found that argument a bit moot.
How did you resist Qt?