The numlock is not a defined modifier that [GDK/GTK passes to the application](https://docs.gtk.org/gdk3/flags.ModifierType.html), so Geany may not be able to do anything about it.
It _may_ be mapped to one of the `GDK_MOD[2-5]_MASK` but thats system specific if it does happen, and what it does on a Wayland system is anybodys guess ;-S