For me, gtk_window_move doesn't work on Wayland. It works on X. I don't
know about Quartz/OSX. There are some bugs about gtk_window_move on
Bugzilla.
Are you running Wayland on Linux?
Is it only window-position that does
not work or is gtk_window_move also broken for you?
That said, maybe there is no need for your window to override the window
manager's default position?