You should add --nosyncacls:
imapsync --nosyncacls --syncinternaldates
--host1 server.gtds.lan --user1 yourAccount --password1 yourPassword
--host2 zimbra.gtds.lan --user2 yourZimbraAccount --password2 yourZimbraPassword
ZCS' IMAP implementation has no acl related capabilities. Hence don't waste efforts with that.
- Gert