The USA GSM carriers are AT&T and T-Mobile.
Sprint and Verizon are CDMA.

FWIW, GSM phones are generally more innovative, as the GSM standard requires "any device with a valid SIM" to be able to use the GSM network, while CDMA is totally under the control of the carriers, who can decide what devices are allowed to connect to their network.

All of this will be moot in 6-7 years when we have transitioned to LTE anyway.