I have been using both professionally for at least 15 years and IMO without a doubt Qt is so much easier to use, read and work with it’s not even a comparison.
I also use Qt professionally, and it is indeed an excellent GUI library. I have absolutely no complaints with how well it is designed and how easy it is to use, and I am consistently amazed by how beautiful the results are, especially with desktop environments like KDE Plasma.
My complaint, which is really a deal-breaker for me, is that Qt effectively forces you into using C++ and Python and/or QML+Quick. For the non-professional software I develop, I want my apps to be scriptable by end users, and I do NOT want to force them to choose between only Python or Quick as their scripting language. For building scriptable, truly cross-platform GUI apps, Gtk is the only game in town.
Gtk is much harder to use only if you are coding in C, because it depends so heavily on the C preprocessor to hack together the infrastructure that C++ has built-in. But because it is so easy to bind scripting languages to Gtk, you only need to program a few very core features in C, the rest you can program in any scripting language of your choice. This very important feature I think is a worthwhile trade-off for making it harder to code in C, especially if you are able to code the larger portion of your application (which is almost always the case) in a scripting language like Lua or Scheme. (Although I admit, most Gtk scripting is done in Python, just as it is with Qt.)
Plus in comparison to Qt there are almost no commercial outfits using Gtk professionally and selling products based on it.
Perhaps, but I would point out that both Canonical and RedHat (now IBM) are both heavily invested into developing Gnome, and I believe most of the paid Gtk development has been funded by these two companies.>