To secure the gallery pages, you would need to apply the page access restriction code to each of the gallery design index pages
In power gallery there is a folder under the galleries directory for each design.
In each of those folders is an index.php page. The user registration page access rule needs to be applied to each of these index pages.
See the tutorial "How to Restrict Access to pages on your site" In the how to section of the User registration solution pack support page: