@holy,

Currently you can only allow the user to create new galleries through the backend, all you have to do is create a user for them and grant them the right access in the setting > user permissions tab.

If you want a greater control on the permissions you can use a roles plugin and be more specific on what the user can do. The content type is “ims_gallery” and once you install the roles plugin you will see all the permission you can grant to the user.