Is it necessary to enable IMAP in gmail before using GmailBackup?

Yes, the IMAP access is necessary to run GMail Backup. If you don't have the IMAP access, you will get the following message:

Error: IMAP: [ALERT] Your account is not enabled for IMAP use. Please visit your Gmail settings page and enable your account for IMAP access. (Failure)

You can see more at http://www.gmail-backup.com/incremental-backup#comment-164