To detect harmful gases
The MCU I have chosen is good for my project


The ESP8266 only provides one analog input, you have to either multiplex it, or use another MCU, for example the ESP32 would be a great option!


  I didn’t find the till what distance these sensor can sense the gases in the environment.


Gas Sensors do not work with "distance" they work within a volume of air. To detect the gas concentration in a room for example, you have to know the weight of the gas (lighter or heavier than air) and then place the sensor either on the floor or at the ceiling. If you just want to "detect" gas, it would be good to create a airflow, eg. ventilation.