Gmail Blog's explanations:
Two weeks ago, we introduced our Gmail app for iOS. Unfortunately it contained a bug which broke notifications and displayed an error message, so we removed it from the App Store. We’ve fixed the bug and notifications are now working, and the app is back in the App Store. For an overview of what’s available in the Gmail app for your iPhone, iPad or iPod touch, check out this blog post.
Now you visit the App Store to install Gmail App to your iPhone or other iOS 4+ devices.