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?
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