Welcome to the Advanced Custom Fields NextGEN Gallery Custom Field plugin page. As the name suggests, this script is an extension for the Advanced Custom Fields WordPress plugin.
This script works only in ACF version 4.0.0. and up. Version 3 is currently not supported.
Besides that this script can be either used as a WP plugin or as a theme include.
This plugin provides an extra field for the Advanced Custom Fields plugin to support the NextGEN Gallery plugin. This makes users able to link galleries to posts, pages and custom post types.
This add-on will work with version 4 and up of the ACF plugin.
If you discover any bugs or have any feature requests, I would like to refer you to the Github Repository of this script.