EDIT: They changed their tune to just "there must be a lot of traffic for this update".
[1]: http://lifehacker.com/5965550/gmail-for-ios-gets-a-completel...
[edit] Direct Link: https://itunes.apple.com/app/gmail/id422689480
Edit: tried the link above and still get the same error (and I've never installed it before, so this should be a fresh install).
edit: 2 minutes later, can't even find the app on the app store.
you have to use the direct link
https://itunes.apple.com/app/gmail/id422689480
obviously a search for gmail should work, but as you discovered, it doesn't, unless Apple have fixed it by now.