A visit to http://appgallery.appspot.com gives the following message
The App Gallery is no longer available. This page should redirect you to the App Engine docs. If not, please click on: App Engine Docs
According to a post on a thread on the GAE Google Group, Ikai Lan(Google Staff) clarified
... App Gallery was something that was really awesome in App Engine's infancy, but App Engine has since matured a bit and it's really not the best way to showcase App Engine's capabilities. We've decommissioned it. If you're interested in App Engine happenings, you'll definitely want to regularly check out our Reddit page:
The Reddit page is located at http://www.reddit.com/r/appengine
The gallery was apparently pulled down without prior notice and users at the thread can be seen voicing their dissent.
Hopefully, AppEngine won't suffer the same fate.