But I was unable to find information about how to set up my repo and build process with GTK vendored into the project. All tutorials said that I should just install GTK libraries with my system package manager, but that was not a satisfactory answer for me.
After some hours fiddling around stackoverflow, adding flags to my GCC invocation and being unable to solve the problem, I just gave up.
Flutter Desktop seems like a compelling alternative to GTK these days.