Right, the 'c' attribute is declared in the core schema. However you
need to add it (and also the other existing attributes) to your
gaPlayer objectClass, either as m-must or m-may.
However I would recommend another approach: Define your gaPlayer
objectClass as 'auxiliary' with your gaXYZ attributes. Then use an
existing object class (e.g inetOrgPerson) as structural object class
and gaPlayer as additional object class for your users:
dn: cn=imavroukakis,ou=Player,dc=gameaccount,dc=com
objectClass: top
objectClass: inetOrgPerson
objectClass: gaPlayer
...
Kind Regards,
Stefan