Hi @dom
Because you are saving / loading data from the options page add-on, you must also remember to specify the $post_id parameter as shown in this doc:
http://www.advancedcustomfields.com/resources/how-to/how-to-get-values-from-an-options-page/
Basically, the get_field should look like:
get_field('gallery', 'option')
Thanks
E