These are orthogonal things. Gtk is for layout and widgets. 2D/3D drawing should be done using an appropriate 2D/3D API, e.g. Cairo or OpenGL. In my case its all 2D and you can draw custom, with Winston, with Gadfly, or with whatever supports Cairo. I once heard that Makie is also supposed to work with Cairo, in that case this would also work.
I know that Simon likes it more to also do the entire widget machinery in OpenGL but my opinion here is that one has a hard time writing an entire UI toolkit from scratch. Gtk is certainly not perfect but it offers a lot. IMHO the API is nicely designed and being a C API makes it easy to use it from Julia. There are some downsides but what is currently lacking most is some manpower on the Julia Gtk package.