I didn't find the answer to this question through searching, and figured it out myself, so I figured I would post my solution in case anyone else had this problem.
I've got several domains (about 15) on my Zimbra server, each with different requirements for the Global Address List. Specifically, I have one domain, that needs to see only four domains of users for the GAL. Every solution I've found either gives 1 domain (Internal), everyone (External, with dc=com for LDAP Base), or 2 domains (Both, using one of the domains as LDAP Base).
I was able to accomplish this by using external, pointing back to the Zimbra server, adding the domains I wanted into the LDAP Filter, and setting the LDAP Base as dc=com. For example, here was my LDAP Filter for the four domains:
You can add as many zimbraMailDeliveryAddress entries as you need.
Hope this helps someone else as well.