In GroupWise 8 there's a new functionality added to the rules. There is a new concept called the vacation rule. Problem was with the former GroupWise 7 client users could forget to turn it off (see here for a workaround). In GroupWise 8 there is a special pre formatted rule to prevent this from happening, and even better, the vacation is automatically added to your calender so your co-workers know you're not in the office.

In the GroupWise 8 client go to tools → Vacation Rule which will give you this window:
As you can see there's an end date of your vacation and you can select whether you also want to reply to external users. You might want to change the message though ;)

Also note that the vacation will be added to your calender: