The easiest way would be to check and make sure all of your contacts are of "Google" type rather than "Phone" type. You can go to
www.google.com/contacts and see if they are all listed there. If so, this will be very easy for you. I'm just going by memory here for what I did on my iPad, but it should be pretty close:
On your iPhone, you'll go to Settings, Mail, add new mail account, and choose Exchange (
NOT Gmail) option. Then use your full gmail address, and type
m.google.com for the domain. Now you can automatically sync your Contacts, Calendar, etc with Google. I tried this on my iPad and now everything is synced with my Android phone. Very awesome.