It's just too bad Gtk devs have been progressively removing keyboard input features from Gtk3 and 4. The idea of Gtk as a generic interface is long dead. These days Gtk really just is whatever GNOME is. Which means things like being able to paste filenames into a file->open dialog (gtkfilechooserwidget.c) have been intentionally broken and left to rot since 2014.
If you want to make an application that uses keyboard input, don't chose Gtk anymore.
Ironic, given that in the dim and distant past (i.e. version 1.2) Gtk's (admittedly butt-ugly) file dialog could be driven very easily from the keyboard, even supported effortless wildcard filtering and shell-style tab completion.
If you want to make an application that uses keyboard input, don't chose Gtk anymore.