I think that every dep file should include all of the required deps (recursively) for an extension to work (but not necessarily include optional deps) and not assume that something (Xorg, Xfbdev, etc) is loaded.
I need to spend some time on this for gtk3 to ensure it is correct, but in the meantime, I uploaded your suggested dep file that allows the gtk3 start-up script to work with Xfbdev.