Do apps included in the gApps zip get updated via Google Play?
Short answer: Gapps will be updatable via Play Store. You can set them to update automatically if you will (personally, I don't. I always try to find a free wifi hotspot to do updates ;)

More specific answer: The Play Store app updates itself silently in the background; you will never be asked for an update.

Longer answer: Gapps must be installed separately because the apps in the Gapps set are not open-source. It was a compromise reached between Google and ROM devs. In essence, ROM devs are free to create their own version of ROM as long as they exclude Gapps, but Google then went further to provide an installable Gapps.

That is why Gapps' updatezip are rarely updated; after all, Gapps still have the ability to be upgraded via Play Store.