Hasta La Motion!

I´d like to ask the experts how I could have a gizmo that runs a function when the gizmo is loaded. I need that to happen not only when I execute a command to launch the gizmo for the first time but also when I save the file and reload again.

The idea is that I have a select box, whose values need to be filled with the names of the files on certain directory. I know how to do that by pressing a button, but I don´t want the user to press the button whenever he/she opens the nuke script.

Is there any solution to this?

Thank you in advance!