Get Google Apps on your mobile device
Android iPhone BlackBerry Windows Phone  
Options for access
Use your iPhone's native apps Set up your iPhone with Google Sync to access your Google Apps mail, contacts and calendars using your iPhone's native applications (your Google Apps administrator must enable Google Sync for your domain).
Download Gmail for iOS Alternately access Gmail using our Gmail app for iOS, which combines features from Gmail's mobile browser and iOS. It's available for free from Apple's Apps store. Learn more
Use your phone's mobile browser Access other apps such as Docs and Tasks from your iPhone's browser app. Just point the browser to, touch the app you want to use, and sign in to your Google Apps account. 

What other Google Apps can you use on your iPhone? Visit Google's iPhone Help Center.