I think all you need to do is:
1) Configure GateIn with your openLdap according to instructions from wiki pages like https://community.jboss.org/wiki/GateInwithLDAPasadefaultuserandgroupstore or https://community.jboss.org/wiki/GateInwithLDAPinreadonlymode according to your use case.
2) Configure GateIn with CAS according to instructions from GateIn reference guide (I think you already made it based on your comment)
This should be sufficient and after doing both steps, you should be able to login into GateIn through CAS with OpenLdap credentials.
hope this helps,