Lost all my iPhone contacts I wanted to send my iPhone contacts to Google mail and transfer back to my new Samsung Android.

After changing iTunes setting "sync address book contact"-> "sync google contact", no new contact appear in my new Google Mail account and all my contacts on iPhone have been ERASED.

Why has this happened?
You have changed the settings in iTunes to sync from Google contacts rather than sync from Address Book, so iTunes has done exactly that. It synced your Google contacts to your phone, and replaced the ones you already had.