Verizon is not moving to GSM. They are planning on rolling out a 4g LTE data network, which is what GSM's next data network will be, however, the voice network will continue to be CDMA for the foreseeable future. So they will be selling phones with CDMA for voice, and "GSM" for data.
AT&T and T-Mobile are the only real options in the US for GSM now and probably for the next couple generations of the iPhone. It will be at least 2010 or later before any of the CDMA providers move to GSM, if they do at all.
They are moving to a GSM based standard for data. So yeah they are moving to GSM. We are very close to the point where data matters more than the voice side. Verizon making a change like that is big. By the time Verizon actually has a GSM based data network up we will probably all be making all of our calls over data networks anyway so the voice carrier won't even matter.