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.
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.