Its confusing to say that GSM will be shut down, as many phones , and there SIM’s or inbuilt equivalent, are "GSM " or GSM compatible, but these phones and SIM’s are also 3G and 4G … which outside the USA , they all use the GSM sim … Some phone companies may have said that their are migrating from CDMA or other 2G, toward “GSM” when they mean toward 3g or 4g…
The shutdown will be of the 2G on 850MHz, So in fact some places a 2g phone may still work because the base station for the area can do 2g,3g,4g on a higher frequency…