I don't want to show up the zimbraMailDeliveryAddress (loginname) in GAL, only the zimbraMailCanonicalAddress should be shown. To achieve this I changed the mapping from accounts to address book with zimbraGalLdapAttr (see GAL Attribute Mapping - Zimbra :: Wiki).
Now, only the zimbraMailCanonicalAdress is shown in the GAL. I think, it's also possible to shown only the alias addresses with zimbraMailAlias=email,email2,email3,....
zmprov mcf -zimbraGalLdapAttrMap zimbraMailDeliveryAddress,zimbraMailAlias,mail=email,email2,email3,email4,email5,email6,email7,email8,email9,email10,email11,email12,email13,email14,email15,email16
zmprov mcf +zimbraGalLdapAttrMap zimbraMailCanonicalAddress=email