Assuming the iCloud address is not already taken, I think the way to achieve this would be to set up the iCloud address as part of your Apple ID account then set Gmail to forward all mail to that account.
By using this for a while, frequent contacts will get the new iCloud address and use that instead (you could even create a signature asking people to update their records) infrequent contacts sending to Google would still get through to you.
Drew McFarlane 