I have configured the GAL with external LDAP server, but while testing it shows "Search Test Successful" and finally "Search Returns no result".
And from the client side also search is not giving any results and auto complete is also not working.
The following is the filter used for searching my external LDAP server.
(|(cn = %s*)(sn=%s*)(gn=%s*)(mail=%s*))
(zimbraMailDeliveryAddress = %s*)
(zimbraMailAddress = %s*)
And for auto complete I am using the following.