Once someone get hold of your Nexus 7 in an unlocked state, they can easily access all apps and datas, such as photos in gallery, contacts, camera, calendar and Gmail messages. So you need an android app that would let you lock specific apps and protect your privacy.
Perfect AppLock is a useful organized app which helps you protect any applications you want with a passcode. After install and open the app, it will let you enter the default password consisting of four digits. The Google Play and Settings will be locked automatically and you can select individual apps and checkmark it to lock.