I really don't have any GDPS experience or expertise, so I can't offer much help. Hopefully you are working with IBM on this project. The best I can do is refer you to a new IBM Redbook (released on December 31st 2005) called "GDPS Family - An Introduction to Concepts and Capabilities" (SG24-6374). Chapter 2.8 discusses cross-site connectivity technologies.
Another Redbook of interest in this area is the IBM System z9 and eServer zSeries Connectivity Handbook (SG24-5444). Chapter 12 covers "Extended distance solutions".
You can find and downloads the Redbooks at the IBM Redbooks web site: www.redbooks.ibm.com/
This was first published in March 2006