How can I sync all of my Email addresses (including Gmail addresses) using iCloud?
Practically speaking, the answer would be no - iCloud doesn't "sync" multiple email accounts (this is built into iOS). @me.com email address is just like gmail - it is just another email account. If you really wanted to, you could have all your gmail forward to iCloud's @me.com email account, but that wouldn't be practical.

To have your Gmail accounts synced into your iPad, open Settings, on your iPad, tap Mail, Contacts, Calendars, select Add Account, for a Gmail account select Gmail, and enter your information in there. Repeat the process for every mail account you have. (You should notice your iCloud account in the list of email accounts when you first open it)