LTE is not GSM based, despite what many ignorant articles say.
It's a totally different network and radio technology than either CDMA or GSM, but interfaces have been designed to allow carriers to use it beside their legacy systems.
So no, Verizon is not replacing their CDMA system with GSM. Both they and ATT will certainly keep their older systems running for many years. A CDMA iPhone would be quite possible at this point.