Whatever post type the gallery plugin uses can also be modified for what capabilities are needed to do what, though doing so cleanly without hacking the plugin can get a little involved, but the possibility exists. A good first step would be to contact the plugin devs and find out what is possible to change in their plugin, or at least find out how the information is stored and what capabilities are needed by default.

Then try to use one of those role and capability plugins to adapt your needs to the gallery’s data schema. If that falls short, look into altering what capabilities are required for various operations within the gallery data. I think the capabilities are stored in the global $wp_post_types array. You cannot alter them until the post type is initially registered of course. Making changes from the ‘wp’ action should be late enough without being too late.