1. OfflineIMAP expects standard IMAP servers. GMVault has special casing to handle Google's wonky IMAP support, along with features like labels and the "All Mail" directory.
2. OfflineIMAP is GPLv2 (or 3, at your option). GMVault is GPLv3.
3. OfflineIMAP syncs to either a local Maildir or another IMAP server. GMVault syncs to its own custom on-disk representation.
4. OfflineIMAP uses a username/password to log in to servers. GMVault uses XOAuth.
5. OfflineIMAP is fully bi-directional by default; local deletion propagates to the remote server. GMVault notes that "manually deleting emails or emails' directories does not prevent Gmvault from working."
6. GMVault can encrypt its own archives. OfflineIMAP cannot.
Basically, GMVault looks like a much less general-purpose tool, but in its specialization, should allow for a much nicer experience for users that simply want a backup for their GMail account.
I'm fond of the versatility that OfflineIMAP gives me (I can restore my mail to any IMAP server, not just GMail; I can access the local Maildir with other applications like Dovecot and Mutt; etc.), but excited about the possibilities of GMVault for friends and family.
"Do you have your GMail backed up? No? Here, let me install GMVault for you..."
As for myself, I just imap my Gmail accounts and don't use those features exactly to avoid that kind of lock-in. I've used too many email services and clients in my lifetime to assume Google is the final solution. And safeguarding my mail offline with imap works just fine.