It does not appear that the gallery field works when not logged in. I’ve tried everything I can think of here.
Your bet for doing this if you want to allow the user to create a gallery on the front end without being logged in would be to give them a repeater field with image fields for uploading multiple images and then to create an acf/save_post action to take the images from the repeater and add them to the gallery field.
http://www.advancedcustomfields.com/resources/acfsave_post/
http://www.advancedcustomfields.com/resources/update_field/
