[the_why_of_y]

GObject is a ref-counted object system that is designed to make interop with all sorts of languages possible across FFI boundaries, including those with runtimes that include tracing GCs; such a thing is not built into C++.

If the GIMP authors would have used C++ in 1995 they would be stuck with a legacy codebase now that would be ridiculed because it's not "modern C++" - a term that has designated about 3 different programming styles during that time.

Even the Qt developers found that 1995 C++ was insufficient for a UI toolkit and augmented it with a custom "moc" pre-compiler and build tools.