When updating a gallery with a new image or deleting an image, is there a hook which gets called?
With a standard image field there's
add_filter('acf/update_value/type=image', 'my_acf_update_value', 10, 3);
You can use the same filter, just replace the type:
add_filter('acf/update_value/type=gallery', 'my_acf_update_value', 10, 3);
All fields extend the same base class, so they also share the same basic filters.