Yup! And so is the Verizon one. AT&T and T-Mobile only support GSM. Verizon/Sprint supports both GSM and CDMA networks.
But just because the T-Mo phone only supports GSM does not mean it is not "truly" unlocked. It is unlocked, plain and simple. Most of the rest of the world used GSM anyway. Isn't there something different with Sprint and their version of CDMA?