GUID Partition Table (GPT) is a supported partition in Intel® Embedded Server RAID Technology II. What is not supported is installing and booting Windows from a GPT partition.
This is not supported in SW (software) RAID due to no support for EFI in SW RAID. However, this feature may be supported in next generation product if system BIOS allows for EFI Optimized Boot option when SW RAID is enabled. Current system BIOS will remove this option whenever SWR is enabled.