[Blondesense]

My understanding is according the their website, Kerdi can be installed over drywall. Many people will use CBU anyway. Just sort of a belt-and-suspenders extra insurance type of thing.
If you are getting a permit and inspection, you might run it by your inspector first. If he's not familiar with the product he may question drywall in the shower.

FWIW, you might ask any additional questions about your shower in either the tiling (under flooring) or the bathroom/kitchen remodeling (under remodeling) forums. The tile guys who know about this stuff tend to hang around there.

 

[RickyBobby]

Jerdi can be installed over drywall with no issue. Cement board is probably your best option. I also believe that it is not necessary to use the green board if you are installing the kerdi, regular sheetrock would work fine. Check the schulter website, it explains what substrate can be used.