According to this wiki arcticle:
ZCS Connector for Outlook Install Process - Zimbra :: Wiki
It looks like you can disable GAL syncing for the Outlook connector. We need to do this for some of our users, but I can't find how to disable GAL sync. Some of our users don't have a GAL so they get a GAL sync failure. We'd like to eliminate this by turning off GAL sync. Does anyone know how to do this?