Are GPT partitioned disks supported with CX3-20?

The 2TB limitation is host based not array based. The array can support LUNs over 2TB but for Windows Server 2003 there are caveats to using LUNs over 2TB.

Read this technet article on GPT:

Frequently asked questions about the GUID Partitioning Table disk architecture
http://support.microsoft.com/kb/302873/en-us

Also search on technet for "windows 2003 gpt".