Since Kameron 1.3.0 you’re able to create password protected galleries and here’s how it works:
1. Go to Portfolio > Add New and create your gallery like you normally would.
2. When that is done, locate Visibility inside the Publish metabox and click on the “Edit” link
3. Choose “Password protected”
4. In the password field write your chosen password, click the OK button and then click the “Publish” or “Update” button
After a user has entered the correct password, he won’t need to enter the password again if he decides to visit the gallery again form the same device.