I recently set up GMail on a client's Apple iPhone 4S using Google Sync through Mcrosoft Exchange. Now that's a sentence I never would have imagined writing ten years ago! Back then GMail didn't exist, and Microsoft and Apple were in a war for the Desktop that Microsoft was comfortably winning. How times have changed!
Back on topic, the Googly Sync set up process went just great on the iPhone 4S. Google recommends setting up GMail using the Microsoft Exchange option in the iOS Mail app, and it's a simple process to execute.
Yesterday the client called me to say that the Sent Items folder had disappeared from the Mail app. I had a look at it today and verified that the GMail IMAP and iPhone's Exchange folder push settings were correct. The Sent Items folder came back when I removed the Exchange account from the iPhone, restarted the iPhone and then added the Exchange account again. The Sent Items folder magically reappeared and Google Sync started to sync the folder correctly again.
I've read a few reports of this happening on iOS5 iPhones, and this 'last resort' course of action (as I'd describe it) of removing and re-adding the Google Sync Exchange account seems to have resolved the issue for others also. Hopefully it's an issue that will be addressed either by Google or in a future iOS update from Apple.
In the meantime if you choose to do this to resolve your own iPhone GMail sync issues, please make sure you back up any contacts, calendars and any other data connected to the Exchange account before deleting it from the iPhone.
