Apps that haven't been updated are frequently the first to experience issues after an update. If you haven't configured Gmail to auto-update, you'll have to manually install any outstanding Gmail and other app updates. Here's how you do it:

1. Tap App Store from the Home screen.
2. Scroll down to the bottom of the App Store main screen and hit the Updates tab. Wait a few seconds to get a list of all applications that need to be updated.
3. Next to the Gmail app, tap the Update button. This will install the application's pending update.
4. If there are numerous app updates available, touch the Update All button in the top-right corner of the screen to install them all at once.