The TargetAddress is a mandatory address and cannot be excluded from publication. When using Collaboration Services to set-up a GAL sync it it the targetaddress attribute that points back to the original forest and without this attribute Collaboration Services will not function correctly.
Most other attributes can be excluded from the sync by editing your publication going to the "Select Attributes" page, expanding the object class you want to exclude an attribute from and unchecking the check box. Should you try to remove a manditory attribute you will get the following error. "Attribute 'AttributeName' is mandatory and cannot be excluded!"
