I know of nowhere that Verizon is GSM. The only phones they support that do GSM (and CDMA) are their world phones meant for roaming outside the U.S.
For a 3G phone to work with T-mobile's 3G it must support *both* 1700 and 2100, not just one or the other. 1700 is used for uploading and 2100 for downloading. So getting the iPhone to support 3G on T-mobile is a bit more complicated than it looks.