The reloadable property in the gdexample.gdnlib file must be set to true for the Godot editor to automatically pick up the newly added property.
However, this setting should be used with care especially when tool classes are used, as the editor might hold objects then that have script instances attached to them that are managed by a GDNative library.
Changing to godot_nativescript_register_tool_class in GDSercomm should also be done
reloadable = false did the trick
This note on https://docs.godotengine.org/en/stable/tutorials/plugins/gdnative/gdnative-cpp-example.html is what we needed
Changing to godot_nativescript_register_tool_class in GDSercomm should also be done