I am assuming you configured your LDAP settings in etc/gerrit.config.
If you start typing the CN of the group you want in the Selected
External Group field and click "Search", Gerrit should find it in LDAP
and display the full DN of the group.
For example, if I type "Engineers" in the Selected External Group
textbox and click search, and Gerrit displays
"CN=Engineers,OU=Roles,OU=Groups,DC=ad,DC=foobar,DC=com" above the
textbox.
-Nick