If you want to do failover for LDAP servers, you need a load balancer to do it. The load balancer will then redirect the connections appropriately depending on which server is alive. It can either be active/standby or active/active and load balance the amount of connections, eg: roundrobin would redirect 50% of connections to one server, and 50% to the other.
This isn’t the job of LDAP, so if you want to be doing load balancing or failover (active/standby), then you need to configure that separately from Gitlab or any other service requiring access to your LDAP servers.
This link will help get the idea: Load balancing LDAP Servers - Loadbalancer.org