Hello!
Please update gallery plugin to the latest version. We have fixed some problems of posts option in new version of plugin.
In order to delete post from gallery – go to Posts tab -> scroll to the bottom of the page. There you find table with added posts – check the post or page you need and click on “Remove selected” button over the table – http://prntscr.com/9t1nav
If you will have any problem with this – Please contact us through our ticket system. We will check what is wrong and will help you to solve the problem.
.
.