Google have launched there free Google Sync service, that is a Microsoft activesync service for gmail.

http://www.google.com/mobile/products/sync.html#p=default

However, in the previous version it only supports sync for contacts and calendars, that without email sync. Last week, they upgrade their service to support gmail sync. Since iPhone support push notification on active sync email account. It automatically add push notification support to your gmail.

To use the service, simply setup your gmail account following their instruction:

http://www.google.com/support/mobile/bin/answer.py?answer=138740&topic=1...

If everything works, you can remove your old gmail mail setting in your iPhone that use IMAP protocol.