Does iCloud work with Mail and a Gmail account, or do I need to necessarily create a .me account and then forward the mails from gmail to .me ?
If you have a .mac or .me account, you will use that as your iCloud email address. Otherwise, you will get a new mail account. You can either forward your gmail messages or use two different accounts (or not use iCloud for mail).
In answer to your question to completely sync the 2 accounts, the answer is yes. Your .me or .mac account will sync to iCloud.
Your gmail account will sync to Google's "cloud" if you enabled IMAP which automatically syncs all of your gmail.
If you have POP mail enabled for gmail, then those messages exist on your hard drive.
I use gmail via IMAP and I have 8 years of mail messages synced/stored in my gmail account. You can check your gmail settings to see which one is enabled on your gmail account.