You can call [`gtk_label_set_angle(label, 90)`](https://developer.gnome.org/gtk3/stable/GtkLabel.html#gtk-label-set-angle) to make the tab like that. It would probably be prudent to do it from a callback on [`GtkNotebook::page-added`](https://developer.gnome.org/gtk3/stable/GtkNotebook.html#GtkNotebook-page-ad...) so it can be automatically applied to each tab like those added by plugins.