In DpiApp, setting a passcode/password to your gallery can protect your gallery from public view. People without your passcode will not be able to view your gallery, only your client who has your gallery's passcode can view the images of this gallery.
To set a passcode for a gallery, go to this gallery by clicking the link of your gallery title in the gallery home page, then click the 'gear' button, you should see a form with a 'Gallery passcode' item. You can then modify this passcode content, and click 'Update Gallery' to take effect.
After you change your gallery's passcode, all previous share link of this gallery are invalidated immediately. Meaning people holding your previous share link of this gallery will not be able to view this gallery anymore. If you still want them to visit this gallery, you need to send them a new share link.