Why do I need a me.com email account if I'm already using gmail?

Your emails are already backed up/saved on the GMail servers, iCloud doesn't touch or have anything to do with them.

iCloud backup doesn't back up your emails, it just offers you a free @me.com email. Emails stored on this account take up space on your iCloud storage.

If I were you, I'd create the @me.com email. I used to use GMail as my main mailbox, but the @me.com service is better because it pushes emails to your device rather than using fetch.

 

Gmail has push too if you use it with Exchange.