Hi @mg_strategia
Easy. All you need to do is swap the add_post_meta function for the ACF update_field function, you can read more about this function here:
Please note that the parameters are different.
To save a gallery field value, just save an array containing all the image ID’s
To get the save function to work correctly, you will need to use the field_key instead of the field_name. This is explained on the doc page too.
http://www.advancedcustomfields.com/resources/functions/update_field/
Cheers
E
