I have recently updated my Nexus 7 to Android 5.0.1 and I found that the Gallery app is missing. Is it possible to restore it?
The gallery app has been removed from Android 5.0 Lollipop. You can search google play for a gallery app (cyanogen team has one), or you can use the photos app that comes with Google+. This is what Google wants you to use.
The Photos app is actually just part of the Google+ app (and therefore part of the Google applications suite) with its own icon in the app drawer. It identifies itself as a photo-handling app and can work with whatever camera app you have installed. Photos has also been updated far more often than the Gallery app has as well, meaning you're more likely to have updated design and features.
Ryan Conrad has already explained the reason and the alternative, but if you really want to use AOSP (Android stock) gallery, then you might be interested to check these apps, which are based on AOSP source code: