GSM is a 2G standard [0]. Modern phones are pretty hard to put into a single bin. They often support 2G (GSM[TDMA]/CDMA/GPRS/EDGE, etc), 3G (UMTS[W-CDMA]/CDMA2000/EDGE/HSPA, etc), and 4G (LTE/OFDMA/WiMax, etc).
A modern US iPhone or Android phone is perfectly capable of moving between every major carrier on every technology (its almost always LTE anyways). Every major US carrier uses SIM cards.
[0] https://en.wikipedia.org/wiki/GSM "The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation (2G) digital cellular networks used by mobile devices such as mobile phones"
A modern US iPhone or Android phone is perfectly capable of moving between every major carrier on every technology (its almost always LTE anyways). Every major US carrier uses SIM cards.
[0] https://en.wikipedia.org/wiki/GSM "The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation (2G) digital cellular networks used by mobile devices such as mobile phones"