Can you use xiaomi gateway and xiaomi zigbee sensor in America? Can it be paired into MI HOME app or be used in homeassistant?
It can be used anywhere if you follow the instructions 
Really? I just want buy some mihome devices and put them in my home in us,But I’m afraid that they won’t work in other country. So in fact I can use them in US(both mihome app and homeassistant)?
Yes it’s works. Good idea to use Zigbee2mqtt: getting rid of your proprietary Zigbee bridges (Xiaomi, Hue, TRADFRI) instead xiaomi gateway.
As far as I know, Z-Wave is country-specific but 2.4 GHz Zigbee is not.