All of the Google-branded services - Gmail, Google+, Maps, Play {Store|Books|Magazines}, Music, Talk (Hangouts) and Chrome - are indeed "apps" that can be updated separately from the OS through the Play Store like you'd update any other app (except the Store itself, it updates automagically with no notification).

However, all the unbranded parts - Messaging, Dailer, Gallery, Camera, Browser, etc. - don't register as user apps (they also live on /system which is mounted read-only except during OS updates) and need a proper system update to be updated.