Well, it would cause problems because it will try to return an array of images. On the other hand, if you used the code at the bottom of the gallery doc page http://www.advancedcustomfields.com/resources/gallery/ “Create a WP gallery” this returns an array of attachment ID values and you could use those with wp_get_attachment_url(). However, I’ve never tried to add files to a gallery field, have you? Will it allow files that are not images?
It is most likely causing warnings in the background. I’m pretty sure that when the gallery is retrieved it uses the php function getimagesize() and using this function on non image files usually causes warnings. They generally won’t be a problem unless you turn on error display.