I had a support-ticket in April with a similar problem. I my case i had my GAL set to an external LDAP-server (our sites User-db/ldap). When this setting were used, any *local* resources were ignored (and because I did not have any resources in external ldap => nothing showed). (zimbraGalMode: ldap)
What support did to solve my problem was (please be carefull. I have no knowledge of what this may or may not do to your system!):
$ zmprov desc -a zimbraGalAlwaysIncludeLocalCalendarResources
When set to TRUE, GAL search will always include local calendar
resources regardless of zimbraGalMode.
$ zmprov md exampledomain.com zimbraGalAlwaysIncludeLocalCalendarResources TRUE