LDAP Authentication GitLab

nano /etc/gitlab/gitlab.rb

 gitlab_rails['ldap_enabled'] = true

###! **remember to close this block with 'EOS' below**
 gitlab_rails['ldap_servers'] = YAML.load <<-'EOS'
   main: # 'main' is the GitLab 'provider ID' of this LDAP server
     label: 'LDAP'
     host: '<ldap server ip>'
     port: 389
     uid: 'cn'
     bind_dn: 'cn=admin,dc=example,dc=com'
     password: '<your ldap password>'
     encryption: 'plain' # "start_tls" or "simple_tls" or "plain"
     verify_certificates: false
     smartcard_auth: false
     active_directory: false
     allow_username_or_email_login: true
     lowercase_usernames: true
     block_auto_created_users: false
     base: 'ou=Users,dc=example,dc=com'
     user_filter: '(&(objectClass=inetOrgPerson)(employeeType=developer))'

     attributes:
       username: ['uid', 'cn']
       email: ['mail']
       name: 'gecos'
       first_name: 'givenName'
       last_name: 'sn'

     lowercase_usernames: true
     group_base: 'ou=Groups,dc=example,dc=com'
     admin_group: 'sudoers'
     sync_ssh_keys: false
#    admin_group can be your any other desire grouple and it can be only one.
#
#   secondary: # 'secondary' is the GitLab 'provider ID' of second LDAP server
#     label: 'LDAP'
#     host: '_your_ldap_server'
#     port: 389
#     uid: 'sAMAccountName'
#     bind_dn: '_the_full_dn_of_the_user_you_will_bind_with'
#     password: '_the_password_of_the_bind_user'
#     encryption: 'plain' # "start_tls" or "simple_tls" or "plain"
#     verify_certificates: true
#     smartcard_auth: false
#     active_directory: true
#     allow_username_or_email_login: false
#     lowercase_usernames: false
#     block_auto_created_users: false
#     base: ''
#     user_filter: ''
#     ## EE only
#     group_base: ''
#     admin_group: ''
#     sync_ssh_keys: false
 EOS

Keep in mind, only employeeType=developer can have access to gitlab, this is the filter I have set, you can adjust accordingly

gitlab-ctl reconfigure;
gitlab-ctl restart;

You can login with ldap user account that is in the sudoers group and it will automatically grant with admin status