The GroupWise security object 'All User Access' might have permissions set on the Calendar.
When migrating the calendar with the option to migrate the ACL as well, the following might be displayed in the log:
10:58:10 DEBUG: PstTgt::ProcesACL - ACL: ~DEFAULT 1
10:58:10 DEBUG: PstTgt::ProcessACL - Adding ACL rights for ~DEFAULT
10:58:10 ERROR: [5004-24-277-80070057] Unable to process acl for '(null)' on folder 'Calendar'
10:58:10  The parameter is incorrect.
10:58:10 DEBUG: PstTgt::CreateFreebusyDataFolder()
10:58:10 DEBUG: PstTgt::createFolder(name=Freebusy Data,class=NULL)
Looking at the Calendar properties in Outlook after the migration reveals that no ACL object equivalent to <All User Access> has been set.
The <all user access> object cannot be mapped to an email address which is why GroupWise Migrator for Exchange (GME) cannot set the ACL entries on the corresponding folder in Outlook.
The only workaround at this time is to change the proxy access settings in GroupWise to reflect a user object that exists in AD/Exchange.
Furthermore, the Development team has logged enhancement request CR94454 in order to introduce an optional setting to map the GroupWise object '<All User Access>' to an Exchange group/user/object.