Hacked Gmail-reset pass-fowarding to Icloud
iCloud (or iCloud Mail) itself can't access mail accounts on other providers via IMAP (or any other means). But I see at least two options how somebody with temporary access to your Gmail account can try to put a tap on your emails:


By adding your Gmail account as an additional account to whatever mail application he uses. This only works while your Gmail password doesn't get changed (so you should be safe here)
By setting up a mail forwarding rule in Gmail which forwards (copies) all your incoming mail to another email address. To verify and disable, please look inside Gmail (don't know the details and we are not a Gmail support site)