I have been looking for an answer to this question, I have a MS exec that has just been hired in a company that I support. He wants his calender and scheduling to work with the outlook 2013 client. They currently have a Groupwise 2012 sp2 server installed. I have looked, but have yet to find an answer.. I just love the MS pricks..
POP/IMAP is just for e-mail. It might be possible to install the GW2012 client and OL2013 on the same machine, to where Outlook essentially uses the GW2012 client as a "plugin" to talk to the GW backend. This would give e-mail, calendar, and contact abilities to Outlook.