Welcome to the forums. Sometimes it helps to know what version you're running, you can find out by running "zmcontrol -v".
The GAL should be internally created by Zimbra, you shouldn't need to modify that unless you have a specific reason.
Now, your LDAP server is configured differently than mine, but here is how my auth is set up:
What is the error? If the Test works during the authentication config, then it should be working fine. Check your error logs in /opt/zimbra/log to see if there are any issues.
LDAP bind DN template:
LDAP URL: ldap://ldap:389
LDAP filter: uid=%u
LDAP search base: ou=people,dc=domain,dc=com
Use DN/Password to bind to external server: No