Captions=Filename in Image Gallery

I’m setting up several image galleries in a single page. The images are available for both download and potential licensing. I want to have the filename show up in the spot of the caption. I’ve got over 800 images (some used in multiple galleries) and I’d prefer not to have to enter each of those captions in the gallery setup. I’m not afraid of putting something in the functions file in my child theme to accomplish this but just need to know what I need to hook into. I looked at the code for the Gallery shortcode but couldn’t tell where it pulled the caption or how to change it to the filename if the caption isn’t entered.

Any help is appreciated!